My advice is always go with the cheapest.
These players are too dependant on the chips. Philips dvp642's chip doesn't support QPEL. You may try dvp720 however it is not sold in North America. RJ800 seems better. But who knows? Divx6 is coming. New releases of Xvid come out very often. Chips are not going to catch up with the codecs. So grab a cheaper one.
BTW, I have a dvp642. It plays most of my stuffs, DVD, VCD, SVCD, Divx, Xvid etc. Only drawback is I have manually convert all idx+sub subtitles into srt. It plays crappy color of an 1.0.2 Xvid file too.