Guess I might as well kick this debate off. For me, it's Bonaire hands down.
I did my first trip to Coz in May and I was not really impressed. I go to dive, eat and sleep. I could care less if there was not a night club or tourist store in 500 miles. Bonaire is a lot more sleepy than Coz. No doubt there will be those that dissagree but I never did find what I considered a good restautrant. To me, they were more interested in a one time tourist and less concerned with quality food or service than most any place I eat in Bonaire. You can't walk 20 feet without a vendor harassing you to buy something in Coz, this type of bahavior is no where to be seen in Bonaire. There are plenty of shops, just no one bugging the crap out of you.
The diving in and of itself is pretty similar as far as life and water clarity is concerned. From the spots I dove on Coz, I saw a lot less soft corals and hard branching type corals like elk horn and stag horn than I am use to in Bonaire. Fish life was similar, mostly small and med size fish. I did see more turtles in Coz. The dive style in Coz is mostly fall off the boat, follow the DM in a fair current, surface, do a SI and repeat. Shore diving is limited in Coz. Our typical dive profiles on Coz started around 90 ft, worked up to around 50 ft for around 45 mins with a 3 to 5 min safety stop. We would do a 1 1/2 hr SI and then a similar profile with a max depth of 50 ft.
The style of diving on Bonaire is totally different. You drive up to a site, suit up and do your own dive, no DM or guides. My typical dive on Bonaire is a shore dive to 40 ft for 70 to 90 minutes followed by a 30- 45 minute SI and another dive of similar profile. 100+ ft deep dives are possible but I choose not to do them. My prefered dive style is to go slow and closely look over coral heads taking photos. The currents and the need to stay with the DM in Coz forced me to abandon my normal dive style and do a more run and gun approch. Boat diving on Bonaire is different as well. They moor the boat to a buoy and allow you to follow the DM if you wish or do your own profile. They typically limit your time to around 60 minutes. Due to the extra cost of more boat dives we opted not to do more dives while in Coz. We get the unlimited shore package when on Bonaire so we typically do 2 morning, 2 afternoon and a night dive about every other night, each dive 70 to 90 minutes long.
In my opnion, if you want to dive 2 or 3 dives a day and party at night go to Cozumel. If you want max bottom time, to set your own dive profiles or to do much photography go to Bonaire. I have several resorts to recommend on Bonaire with wide price ranges.