I choose the Denon over the Sony for the following reasons:
Audyssey EQ as opposed to a proprietary EQ system.
Network functions, including firmware updates.
HDMI to HDMI scaling (Sony only allows scaling from analog inputs).
Built in HDCD filter.
GUI can overlay HDMI source inputs (I don't believe Sony can do this).
Actually the Onkyo 875 has much better scaling abilities due to it's HQV Renon chip. The reason I didn't choose it was:
Popping reported in speakers through the course of normal operation.
Runs very hot.
No GUI (it does have an on screen display, but it is the older varient, not graphically enhanced).
No network functions.
Can't scale HDMI to HDMI (Onkyo also only allows scalling from analog inputs)
Then of course there is the Yamaha 3800. Feature for feature it is the most comparable to the Denon. However it didn't allow firmware upgrades over ethernet connection and I was more familiar with Denon products.
That's my take on the receivers being discussed. I chose the Denon 3808 and couldn't be happier. You can read my review here:
http://forums.audioholics.com/forums/showthread.php?t=38035
Bear in mind, all of these are great choices. You can't go wrong with any of them. They all have slightly different features. For example, the Onkyo 875 is Ultra THX 2 certified where the Denon is not. I really wanted some of the THX post processing options, but other features on the Denon were more important to me. There will be trade offs for all of them. Just choose the one that fits your needs the best.