My wife and I just got back from an eight night trip to Belize last week. We spent five nights on the mainland, in Placencia, then three nights on Ambergris Caye.
Since it was our first trip to Belize, I think that it was the right thing for us to split up our nights in two seperate areas. Placencia gave us the land attractions and Ambergris gave us the wonders that are in the water.
Placencia is a nice little town in the southern part of the country. We got a great deal at the Laru Beya Resort via Groupon. We liked the slow pace of Placencia and its close proximity to the monkey river tour.
The OP mentioned Hopkins. You have to take four miles of very rutted dirt roads to get there. Hopkins is closer to the Cockscomb Swamp (jaguar preserve) though.
Ambergris, at least near San Pedro where we were, seemed like a conjested party town. That is not our style. We very much appreciated the close proximity to the outstanding barrier reef though. We snorkeled Hol Chan, Shark and Ray Ally, and Mexico rocks, each with its own interesting animals and topography.
I heard that Caulker Caye is less congested and more laid back. If we went again then we would probably do four nights in Placencia and four nights on Caulker.