1. Dive Quality: People say Cozumel is way nicer than Kona for scuba, but looking at the videos posted on Youtube, I could not see the WOW effect for Cozumel? I mean, reefs look washed out, there are different species but you don't see much life (like a few fish here and there), etc. Kona videos have much wider range of corals and again much more lively than Cozumel? I have checked out dozens of videos for each, and as I was saying Cozumel looked pretty bleak to me? Ah, also, not to mention Kona has that spectacular manta dive
Both islands have very clear water. Cozumel has stronger currents overall. I only used one dive op there, Jack's Diving Locker, and we didn't drift dive IIRC. While the currents were weaker than Cozumel's, that still meant we had to kick against current in one direction for half our dive. As the dive op didn't offer anything bigger than Al80s, our dives were rather short. I'll address this factor again in the Prices section.
Hawaii does not have soft corals, so the underwater terrain gets a bit monotonous, though there are some very nice hard coral formations. Hawaii does have a very colorful selection of Pacific tropicals, not with the variety you'll find in the Indo-Pacific region, but definitely a different mix from the fish found in the Caribbean. Lots of species of butterflyfish, very colorful triggerfish, more species of morays, and the main attraction: endemic species you won't find anywhere else but the Hawaiian islands. Cozumel has one endemic species, the splendid toadfish, but a full 20% of Hawaii's fish are endemic. Therefore, it's a great destination for fish lovers who have already been diving in other locales and are looking for something different. It's also a great destination for divers who want a taste of Pacific diving without making the full trip across the Pacific.
Overall, I found Maui diving to be more interesting than Kona. Of course the Big Island is called Big for a good reason and the local boats only go so far up and down the coast. One day, after checking lots of other trips off my bucket list, I will try the Kona Aggressor. What would be cooler than a lava night dive? Anyway, Maui has interesting sites offshore as well as Molokini and nearby Lanai, so I've found greater variety there. Like I said, the terrain can get a bit monotonous, so variety is nice. Kauai, on the other hand, is an older island with lots of rain so the water is pretty murky. That may have been my least favorite of the three (haven't tried Oahu or Molokai yet), though the turtle population was very healthy there and it was good for macro. Maui is the only island I've returned to for a second dive trip (and a third), but that may be because I also prefer Maui topside.
What Kona does offer over Maui (and Cozumel for that matter) is the manta night dive. We did it one night and J loved it so much we signed up again for the next night. I prefer to spend my nights drinking and dining and she usually has to twist my arm to go night diving, but I was very agreeable in this case. We had dove with mantas in Yap and it was just ho hum. In Kona, where seven mantas eventually showed up each night, they give you an amazing hour long show, gliding around, swooping in so close they literally brush the top of your head. While I'm not so gung-ho about the day diving there, the manta night dive was spectacular - easily a must-do dive for anyone's list unless they're a manta hater.
2. Prices: Again, people were saying Cozumel is much cheaper / affordable / etc. Again, looking at hotel + dive packages in Cozumel, you can't get anything less than 2.5-3K USD for 6 days dive + 7 days hotel in something like Presidente or Park Royal (basically any 4+ star relatively nice hotel), and diving is around 80 USD per day in Cozumel (2 tank dive) with scuba du or h20. Looking at Kona, you can get Sheraton / Hilton or condos for ~ 200 USD, and dives are a bit more expensive (like 100 USD instead of 80). So, I don't see how Cozumel is cheaper, leave aside much cheaper?
The prices at the Presidente are indeed similar to mid-range hotels in Kona like the Sheraton and the amenities are similar as well. It will probably cost you about the same to eat at the Presidente versus eating in Kona. But the Presidente is the most expensive hotel on the Cozumel by far. There are plenty of lower priced options and that's what makes Cozumel cheaper. The amenities might not be the same, but you can find very decent rooms for around $80-$100/night, or spend even less if you book a package. I'm not sure what you'd end up with for $80/night on Kona, probably a doghouse in someone's backyard.
Not sure where you got your Park Royal pricing. I found rates on their website around $150/day and that was all-inclusive for 2 persons. I don't think there are any all-inclusive options in Kona and that's a big difference from Cozumel. There are plenty of AI options on Cozumel ranging from the very cheap (meals, drinks, and a room for ~$80/night) to the more expensive such as the Cozumel Palace which runs around $330-$400/night per couple. But that's for an ocean front junior suite with a huge hot tub in the living room, not to mention all the food and drink and other included amenities. That sort of room alone at the Sheraton, if it existed, would run you about the same amount or more, and you'd still have to buy food and drink.
Dining and drinking are definitely more expensive in Kona. As there are no all-inclusives, you can either cook your own meals and make your own drinks in a condo to save money (of course you could do that in Cozumel too) or dine and drink out. Cozumel is definitely cheaper if you're dining out and IMO, the food is much better. There are excellent restaurants in Maui and Oahu and even Kauai, but I was not so thrilled with Kona's selections.
Diving is way more expensive in Kona. I'm not sure where you priced the $100 as $125 is more the usual, maybe with a slight discount for multiple days. On the other hand, I pay $85 for what I consider to be the very best dive op in Cozumel, lots more amenities than you'll find in Kona including gear valet service and steel 120s for much longer dives. As I mentioned before, in Kona you'll pay for an Al80 and spend half your dive kicking against the current. Non-breathers in excellent shape may not mind much, but I was hard pressed to get longer than 45 minute dives. With the larger tank and drift diving, I'm able to easily make 60-70 minute dives with much deeper profiles. Therefore, if you price the dives per bottom time, you'll see that Cozumel can be much, much cheaper.
Finally, you'll have to get to the dive shop. Rental cars are just about mandatory in Hawaii and they're not cheap. On the other hand, only a few rent in Cozumel because many/most dive ops will pick you up right at your hotel dock. Besides the time savings and convenience of being picked up en route to the dive site, it's a big monetary savings as well. Of course you'll want that rental car in Kona in the afternoons as well because there are plenty of road trips: volcanos, black sand beaches, kayaking with dolphins, etc. On Cozumel, while there are a few topside attractions (some really ruined ruins and pretty beaches on the east side), to me it's not worth renting a car for a day at most. Again, however, Kona is big. A lot of the day trips are really day trips, impossible to do after a day of diving even if the altitude issues weren't a consideration. It takes a long time to drive to the volcanos and a really, really long time to drive to the other side of the island. Again, that's why I prefer Maui.
3. Flights: Much more painful for Cozumel and definitely more expensive, i.e. you have to fly thru' Houston, waits between flights are like 5 hours, etc.
Kona definitely beats Cozumel for convenient access from the west coast, that's for sure. From the L.A. area I either have to fly into Houston and overnight or I have to make a super-early flight out (a 5:15 flight means getting to LAX at 3:15, which means no sleep!). Even if you can't get a non-stop and have to change planes in HNL, Kona is definitely easier to get to. And while I find the flights back from Cozumel to the west coast to be more convenient than getting there (due to the time difference each way), you have the additional hassle of clearing customs/immigration in Houston.
So what to do?
As you haven't been diving in Cozumel or Kona, I'm going to guess that you're newer to diving. Or at least newer to tropical diving. You probably haven't see the same old Caribbean fish so many times that you crave something different. Cozumel has very excellent Caribbean diving, pretty much everything you could want: clear warm water, lots of colorful tropical fish with some pelagics as well (turtles, the occasional shark), easy drift diving with a choice of excellent dive ops, and the convenience of being picked up right at your hotel dock.
Kona isn't going anywhere for a while. It's the newest of the Hawaiian islands (except for the one that hasn't broken the surface yet) so it will be around for millions and millions of years more. Get your feet wet in the Caribbean and when you get bored with the selection of fish there and want something different, Kona is an easy hop for you. But for this trip, you'll get much more bang for your buck in Cozumel if you plan your trip right. While the Presidente/ScubaDu combo seems like a "no brainer" if money is no object, I'd actually spend a lot more time thinking that through, if not staying somewhere else, at least using a different dive op. But that's for a different thread.
I lived on Oahu back in the 80's, but have only been back once since toKauai in the mid 90's. I find The Islands more beautiful than Cozumel. I prefer Cozumel and The Yucatan for the people and ambiance. I think though it all comes back to the diving for me. In most circles Cozumel is considered world class diving, and Hawaii is not. So, if this is a dive vacation, I think that is the determining factor. Hawaii honestly isn't even on my dive radar. In the last few years I've been to Thailand and Indonesia. We've talked about Hawaii because my wife has never been, but honestly I've never even considered taking my dive gear there if we went. Maybe my thinking is all wrong on this, I'll be watching this thread to see if I'm flawed in my perspective. If I was going on my honeymoon, or to do land based stuff, it would probably be Hawaii. Here are a couple of my Cozumel videos, I'm not sure what you've watched on the internet, but maybe they'll show you something or an aspect you haven't seen. Good luck in your search and have a great vacation wherever you go 
