I just got back from Belize -- getting there on miles is no problem. What programs do you have? It's only 35k round trip saver on United, 30k (off-peak) to 35k (peak) on American, etc. United and American both have short flights from Texas (United's Houston hub or American's Dallas hub). With United, you can often expect a long layover in Houston.

As mentioned above, the real problem is hotels. Anything outside of Belize City is not part of a major chain with a points program.

If you want to go explore Mayan Ruins, go cave tubing, or anything on the main land, then you may want to spend 2-3 nights on the main land and rent a vehicle.

The Radisson is nice, very close to the water taxi port (if you decide to take that to the Cayes) and not far from the Municipal Airport (cheaper flights to the Cayes, closer to Belize City than the International Airport which you'll fly into). The other chain options are Best Western (mentioned) and Ramada.

Just a random tip, but avoid food near the resorts if possible. It's all American-style and overpriced. Every morning outside my resort on Ambergris Caye, a gentleman would come by with a basket full of breakfast burritos for like $2.50 USD. On the main land, there's a million options on the side of the road which are dirt cheap and blow away the quality and prices of restaurants at Radisson for example. If you stay on Ambergris Caye, you can take the water taxi into San Pedro (the local town) and eat delicious food much cheaper.