They are both the same as the B3 except:
B2 has no percussion.
Rt3 is Bigger so that it can accomodate the concaving 32 note Pedalboard like the ones on pipe organs.It has extra stops on the right to the great manual to control the higher pitched pedal sounds.
I would not buy either one. I needs my percussion and I also like the 25 note pedals, so that when I get to church I will feel right at home. The 25 note board is also easier to play.
For those reasons, I would stick with a B3, C3 or the A100 which would probaly be my first choice being htat it has reverb and built in speakers for more variety.
But if you had to choose, I'd suggest the B3 because its probaly worth more beingt that it has that famous B3 Cabinet and it will be easier to move