Ambergris has got very pretty reef diving.. its considerably affordable at about $25usd per dive. This goes to nearby reefs (5-10mins boatride) which are mainly Canyon diving... tacklebox, victoria, cypress etc. Accomodations can be reaaaal affordable. i stayed at Pedro's inn... really nothing much, but good enough since i was out the entire day diving and juz needed a bed to sleep. I would recommend Amigos or Protech (if the the dms i was w are still around though).
CayeCaulker unlike Ambergris, doesnt have nearby reef diving. They offer premiere dives as mentioned, trips to BlueHole, Turneff North/South etc. These trips are excellant and should be taken from CayeCaulker instead of Ambergris as its cheaper (nearer). I would recommend BelizeDivingServices or BigFish. I stayed at Tina's. An excellant backpackers hostel.
i did 23 dives in Belize spending my time on both Ambergris and Cayecaulker. If u are a rather new diver, itd be nice to start off on Ambergris before going for the more expensive trips from Cayecaulker. Ull hopefully enjoy more that way!
By the way, go DIY, its easy... forget about full board packages. u can get meals on Ambergris for like $2.50usd easily. And accomodations, juz surf the web its pretty comprehensive. Cheers!