The 7300 is very superior IMO. It is newer, more powerful, and able to power 7 speakers whereas the 8000 can only do 5 but has the software to run 7 and the connections for an external amp. The 7300 has a much better auto calibration, high quiality internal video scaling, and the latest decoding foramts (PLIIX, DTSES with the different modes). The 7300 also has more connectibilty and is more future ready.
The 8000 has at least two advantages (both are debatable) It has THX Certification, and it is a better looking unit IMO. THe 8000 would make a fantastic receiver in a room where 7.1 wouldn't be an option as long as the owner wasn't the kind of guy who needed all the latest bells and whistles. Also the 8000 should be much cheaper than the 7300 right now. At a futureshop by my house in Canada they have an 8000 on clearance for $2199 but I'd imagine the price would be closer to $500 in the states.