Although the Keys are home to the biggest bonefish and amazing tarpon, Belize offers more bonefish (that are less pressured and therefore more approachable and catchable) and probably even better shots at tarpon - the downside is getting there. It's really the lack of pressure on this fishery that makes it special. Also, Belize just made bonefish, tarpon, and permit catvch and release only, which will make the fishery even better.
There are several great lodges with very different options. Those out on the islands (Turneffe and Ambergris) have access mostly to flats - lots of bones, mostly smaller tarpon, and lots of permit. Those on the mainland have those plus more shots at giant tarpon (especially around the mouth of the Belize River) and better chances for snook.
I recommend trying both options and finding which ones suit you best.