Well to start off the 7900 is last years model that is actually replaced by the 7000bt, and the 6000 is the current years model with not only an ipod connection, but also a usb connection for generic mp3 players, flash drives, etc, etc. Both are great units that have pretty much the same capabilities and specs.
The biggest difference is the bluetooth is built in the 7900, but it doesn't work very well (it makes you sound like a 5 year old girl) but the 6000 actually has the capability to be bluetooth compatible you just need the external box for it.
James (GeddyLee) on this board actually bought the 6000 and he likes it, maybe he will help sway you one way or another.
-steven-