The 804 is brand stinkin new. Came out <i>maybe</i> a month ago. I had the '674, but ended up returning it to get something with preouts (which ended up being the HK 240) Now the Harman Kardon is sold, and I have the 2807. You're going to get a bit truer power with the Denon, but in my opinion, the Onkyo is a little more user-friendly.
Is the Denon $200 better? Maybe. But considering the Denon's been out for a while, you may be able to find it for less than a Bill, in which case, I'd vote Denon.