Between those two, I would go with the Denon. There are two reasons for this. First, there have been reports at sites like this one that some have had reliability issues with their Onkyo receivers recently. Second, Onkyo does not include the original Dolby Pro Logic (DPL) on their audio video receivers, so they cannot properly decode DPL. Either one of those reasons would be enough for me to stay away from Onkyo receivers, so between the two, I would go with a Denon.
In fact, however, I went with Yamaha, which I think is a better value. Pioneer is another brand I would consider for its price/performance ratio.