I've tried all 3, EP175, EP350, EP500 in a similarly sized room.
175 not enough power for my tastes. Depends on your volume. I play mine in the too loud category most of the time. Could be fine for toned down volume though.
350 worked fine for me for about 3 years, but there were certain movies with a lot of extra bass output that would tax the subwoofer.
500 had for a few months now. Sounds great, has plenty of headroom for me, so far, no noticeable problems.