One of the things I like about the B3 is that the volume knob is a regular old output level adjustment (not a volume loop tuner). This gives a lot of flexability into what sort of amplifier you can plug into without an attenuator. It means common guitar amps easily work pretty well with he Burns Theremin.
I also preferred he voice of the B3, but that decision was from youtube and other Internet recordings using PC speakers or headphones, not listening to both in person. A LOT of people like the the Moog. I imagine you can't go wrong with either Theremin.
The amp issue has been covered on many threads before, so search this site for the existing discussions of the topic. Lots is stuff will work well. Guitar amps, keyboard amps, and powered PA speakers, are common options. The only real losers are cheap guitar practice amps. They dont even sound good with guitars! In general stay away from little speakers with a guitar amp. There maybe exceptions, but usually little speakers mean tinny 1970s AM pocket transistor radio sound. Look at amps with a 10 or 12 inch speaker. Reverb is a nice feature too.