While the speakers in the 9100 may be marginally better overall than the speakers in the 7200 I wouldn't let that sway my decision if it means putting you over budget If you choose to upgrade at all in the future, you'll probably choose to upgrade the speakers first regardless of which system you got.
Both systems come with good receivers. Plenty of inputs, and they fully support all that Blu-Ray has to offer. I don't put much stock in the "THX Certification" - it simply doesn't carry the same weight that it did 10 or 12 years ago. The 9100 does have multichannel analog inputs, but since it fully supports Blu-Ray audio via HDMI, those will only come in handy if you have, maybe, an HDDVD player, or even an old laserdisc player that you'd want to hook up that way.
I think the 7200 is a fine system, and with the $150 or so you'll save, that will be just about enough to go pick up a new Blu-Ray player (there are several profile 2.0 players available this holiday season for about $150 one from Samsung, I believe). Don't forget the speaker wire and HDMI cables (from Monoprice.com).