yahtzee-
First of all, Welcome to SB.
The Differences between Dominica and Belize are huge. Likewise, so are the differences between the different parts of Belize. I guess it completely depends on what you are after.
Dominica for us was a wonderful trip. A rugged island with rain forests, streams, and hundreds of waterfalls. The villages are sleepy and safe, and the diving is excellent. It is certainly "off the beaten path". Water conditions may have the potential to be a bit challenging with the associated changes in current flow. Most diving is in the "Scott's Head" area where it's done on the rim of the extinct underwater volcano. If you are into exploring, sightseeing, etc, then the island will keep you busy on your surface intervals. Not much for choices in the areas of food, drink or shopping.
My experiences in Belize deal mainly with Ambergris Caye. 3 trips in 3 years. Its a nice little island with great choices in lodging and food.
http://www.ambergriscaye.com/pages/lodge/lodging.html It is safe to walk the town, or rent a golf gart for the island exploration. The local diving is nice--and really close with sites generally 10-15 minutes away from the dive shops. Surface intervals are generally done back at the diveshops. Diving is relatively easy, minimal current, ranging from 80-100ft on the first dives and 60-70ft on the seconds. Day trips to the great diving of Turneffe Atoll and Lighthouse Atoll (The Blue Hole) are also available, though its about a 2 hour trip each way. All our diving has been with Protech.
http://www.protechdive.com/ They were previously located at the Belize Yacht Club and are now transitioning to the Spindrift Hotel. Likely to be a bit more for you mom to do on Ambergris as compared to Dominica. You've also got the options of day trips to the mainland for Mayan Ruins, Cave Tubing or Jungle Tours.
Either location would be a great get-a-way. Enjoy your time and feel free to send me a Private Message if you've got any specific Q's. Ken :42: