I am glad I came across this thread. I am looking to upgrade from an Onkyo TX-SR609 that has been a solid performer since I bought it about 5 years ago. I was looking at both models today at Fry's. The Denon is on sale for $398 and the Onkyo is $425. I have typically used Onkyo receivers, starting with a TX-SR800 over 10 years ago. The Fry's rep felt the Onkyo was a better receiver but really couldn't tell me why other than stating that the Onkyo was more powerful. I don't think the power output will make that much of a difference as was previously stated in the thread. The sale lasts for a couple of more days. Unfortunately, I don't have the ability time-wise to do an extended test with both units.
With price basically the same for both units, from what I have read, the Denon seems to be a strong candidate at this point.
The receiver is in our media room with moderate use (10-20 hours/week) only for movies and TV - no music. I use an ATV4, Tivo Mini, Roku 3, and a Blu-Ray player all via HDMI feeding a ViewSonic Pro8200.
It appears that the big difference is Audyssey on the Denon.
Any other items to consider between the two models?