All I know is that the BP is for bass and RP is for guitar, I believe that they're very similar if not exactly the same, but I guess they would be more specific towards the pitches produced by the instrument, so the BP would emphasize more on the lower ends. I'm not sure though...they could be exactly the same.
My son and I have got both the RP90 and the BP90. They are very different. Most settings on the Rp90 make your bass sound like a kind of drop B guitar. The only bass sound which can be produced with the RP90 is the Lemmy Kilmister sound. The other way round, the BP90 isn't very well suited for guitar. I guess that the situation is the same for the RP and BP355.