If the extra $200 isn't too much of a blow to your pocket, go with the PB12 (as you said, future proofing). Either one will be more than adequate for your room. I'm extremely happy with my PB10, and I used to have my brother's system (pmbuko on this forum) with a PB12 in this same room. Both provide seemingly effortless bass at any volume and can really plumb the depths convincingly (my PB10 has strong response down to 18hz in my room).

Of course, if I could afford it I'd be all over an EP500.