That's a tough one. Lot's of happy owners of 500s and 600s here. I own the EP500 and am very satisfied with it. Although if I tried out a 600 I might not be.
By the specs, the 600 plays just a bit lower and louder- but it is only "just a bit"- about 3 Hz lower and 2 dB louder. My 500 plays plenty loud and 2 dB is questionably a discernable change. I think the 3Hz lower may add some additional realism to modern soundtracks with really low bass.
If the budget is tight, I'd go with the 500- I don't think you will be dissapointed and it seems to be the best value. If you have the funds and want the best sub Axiom makes, then get the 600.
Your room is a good size, but either sub would do very well.
