Until a few years ago the 350 was the king of the hill for Axiom subs and I owned one for 2 years, there is nothing wrong with the 350 at all. If you have a small to medium room you would be fine.
A lot depends on your listening habits. The 500 and 600 will play much lower and have bigger amps and have a DSP chip, so technically are designed different. Oh, I forgot about the 400, it rocks as well in a smaller footprint.
If you want the hair to stand up on your legs, get a 400, 500, or 600. If money is a concern and you want a great sub, the 350 might be fine.
Warning....."Upgradeitis" sucks.
