The AC one's seem to be the skimpy whimpy looking treadmills that look like they would be better off as a pet treadmill.  Look for ones that shout out quality and you'll probably score a PM motor along with some nice bearings, high density particle board, electronic parts, etc, etc.

Also be sure and google search this site because there are a few posts about treadmill motors such as this .  In that post is a treadmill motor that can still be bought for $9.99 +s/h.