The Denon has better video processing (ABT > Faroudja) and runs cooler while the Onkyo is cheaper, offers the new Audyssey listening mode (requires additional speakers) and has 2 subwoofer outputs. Of those 2, the Onkyo looks to be the better deal if you don't need the video processing and have plenty of room around the receiver for ventilation.
If you don't need the networking capabilities, I would suggest the Denon 2310 or Onkyo 707. They are basically the same as their big brothers otherwise. Between these two, the Denon has better video processing while the Onkyo has pre-outs. If video is important, go with the Denon. Otherwise, the Onkyo allows you to add an amp if you need more power.
Denon is currently on my s**t list because the DVD player I just got had its HDMI output break and my Marantz receiver needs service (HDMI as well), but my parents' have a Denon receiver that works great. My Onkyo receivers are working flawlessly however.
