I am not familiar with these particular models. However, I owned a 3B-SST and now I have the SA-102 (both are basically scaled down versions of your models). I think it's hard to go wrong with either one of them, the choice being eventually a matter of taste. The Bryston may have a tad more perceived grunt (perceived, because technically each amp is pretty powerful). It may also sound "clearer" (the sound is clean and "neat" to a fault). The Plinius on the other hand sounds a little more natural, rich and "liquid" to me. They are both fine amps. Do try to listen for yourself. If you're buying new, try to arrange a home loan with the dealer.
FWIW, IMHO, IMMV, etc., etc.
FWIW, IMHO, IMMV, etc., etc.