Diving off Ambergris is quick and easy with most dive sites being only 5-15 minutes from shore so if you don't want to be stuck on a boat all day then Ambergris is the obvious choice for easy close to shore diving, plus Ambergris is a lot cheaper to dive, compared to Placencia so if you are on a budget then Ambergris is the more economical choice with comparable diving.
$75.00 per 2 tank dive Ambergris Caye
$95-$125 per 2 tank dive Placencia
A typical Ambergris dive scenario is you head out for your first dive and will be in the water within 15 minutes of leaving shore, conduct your dive and then back on the boat and head back to shore to spend your surface interval onshore, that is unless you head out to the Marine Park where the surface Interval is done at Shark Ray Alley (great photo op)
Ambergris Caye offers you the chance to do up to 4 dives a day and a night dive if you wish although there is only one night diving spot which is the cut at Hol Chan (excellent night dive even better if caught on neap tide)
2 dives are normally completed by lunchtime and you have the afternoon to do something else or go on another 2 dives.
Mainland trips are also easy from Ambergris with almost daily trips to Cave Tubing, Mayan Ruins etc
Placencia as mentioned is a lot further away from shore and takes typically 45 minutes to an hour out to the dive sites depending on boat and dive site selection and you will be out for most of the day to do your 2 dives and its 25%-40% more expensive so based on your post about budget then Ambergris is the better fit as the diving is no better off Placencia than it is off Ambergris.
Placencia has better beaches than Ambergris and is also closer to some mainland spots but as far as the diving its comparable to Ambergris Caye.
I know you said don't recommend the atolls BUT I have to mention it and make sure you know that if you choose Placencia or Ambergris that is NOT where the best diving in Belize is located and while both have some very nice dive spots its NOTHING IN COMPARISON to the Atolls of Turneffe, Lighthouse Reef, Glovers and really the diving is night and day.
The diving at those 3 locations blows away anything that Ambergris Caye or Placencia can offer you and while it may be out of your budget it is important that you are informed as to where the best diving is located in Belize so you can make an informed choice especially as you have dove in location such as Palau.
When I operated my resort in Belize I had so many experienced divers that were less than happy with the diving off Ambergris not because its bad it is not, but that there expectations were more after reading about how fantastic Belize diving is and they said they wished they had known before coming to Ambergris that the best diving was located at the Atolls.
When you take into consideration on the Atoll resorts that all meals 3 dives a day night dive etc etc are all included it really is not much more than taking a regular package to either Ambergris or Placencia and the diving is 100 times better.
Also I have to ask where else have you dove other than Palau (great trip report by the way) reason I ask is if you have dove other great places I really have to direct you towards Belize Atolls as I don't feel the diving will keep you happy either off Placencia or Ambergris if you are a really experienced travelled diver. (Of course every diver is different)
If you want to dive the very best of Belize then I have to recommend the atolls of Belize if you just want to get wet and have a nice dive holiday with nice diving then Placencia or Ambergris will fit the bill.
I have included a link to dive sites explanations off Ambergris Caye for your perusal there are also dive site descriptions from other locations also on my site and a poll of the top 5 dive sites off Ambergris.
Dive Sites Off Ambergris Caye
You may also be interested in a video I put together called
Ambergris Caye Is it Right For You
Ambergris Caye and Placencia are nice locations offering Nice diving and both have some very unique attractions ie Whale Sharks Placencia and some Great Nurse Shark action on Ambergris and both locations are best suited to beginner to intermediate divers. for experienced divers that have traveled extensively to other exotic dive locations and have a lot of dives under there belt, then from experience nothing but the Atolls of Belize will keep them happy.
I'm in the middle of a major overhaul of my website and will be adding videos for Placencia and the Atolls shortly so feel free to check back
Best regards
Dive Safe
Gaz Cooper