Either will work. If you are dead set that you need to use Audyssey, then the 105 does you no good - as many have said - Audyssey only works on the digital inputs (as well as analog 2 channel). Will analog 7.1 in sound better than your current setup with Audyssey bypassed - almost certainly, but you still may not like it. My speakers are mismatched as well and my center is very low-end - it's a work in progress. But I still hear a marked improvement using the 105 - and I think it is a better improvement than buying a better center would have been - one day I'll find out. And while Audyssey does something "good", it also is very heavy-handed with the EQ and can sound strange especially compared to the raw unprocessed channels.
Listen to a bunch of Stereo recordings and toggle between Stereo with Audyssey and Pure Direct. If you never find yourself compelled by the Pure Direct mode, it is unlikely you will be compelled by essentially the same thing with even more channels and better DACs.
The other side of the coin - given your 30 day trial / return situation - how big a deal is $700 to you? If that is a lot, then given you are mostly interested in movie sound and already prefer Audyssey processing, maybe your current player is already doing everything you need.