to NICO!
I don't think the '04 Q has many differences vs. the '05. Most of the glitches were worked out by then. I think '05 was just a refresh of the styling, like you said.
The main disadvantage to that series of Q45 is that it's rare. Kind of nice because you won't pass yourself driving down the road - but there isn't a huge pool of internet forum support, aftermarket parts, etc. The M is slightly better in this regard.
For the Q, I would have long term concerns about parts availibility, but in the short term you should be ok.
If you get close to buying either car, check the dealer service history - they can run it by VIN.
Good luck!
Heath