here's my two cents worth. i'm by no means an expert. have been to san pedro two times. first trip was with my spouse, second with my brother. each trip had a different agenda and thus different accomidations / costs / etc.
Ambergris is not like your typical "resort" style trip to the carribean. I've done those too (the all inclusive thing. Based on what I've seen, my take is that San Pedro is much more like what you would have found in the more touristy islands 50 years ago. Now, depending on what you looking for, this can be either great or a dissapointment (for me it was great).
I'd suggest working with a travel agent who specialized in this region. Barb's at Barb's Belize is who I used for my first trip and she was great. I'd suggest you call her and chat rather than try to do your whole search online.
Where did I end up? Spent a week at Victoria House with my wife and had a great mellow week. VH was lovely, upscale, refined, quiet and classy. Priced accordingly (for belize, for USA, it was a great deal). Second trip was with my brother and on a budget. Stayed at Bananna Beach. Functional, fine, comfortable, affordable, busy.
Again, first trip (given what you've said) I'd work with Barb for planning.
Feel free to post follow up questions. Boarders were very helpful in my search and I'll chime in if I have something meaningful to add.