In my experience, Denon builds to a much higher quality standard than Onkyo has. You get more bells and whistles with the Onkyo, but their A/V receivers tend to run hotter and can overheat more easily. This indicates that their amplification section is not as robustly built as the Denons are.
This has traditionally been the case that Denon does not offer as many features, and the Denon bells and whistles outside of standard A/V receiving/amplification are weak. But, the A/V switching and amplification is one of the best in the industry.
I think of Yamaha, Pioneer, and Denon, that Denon has the most well built product. I think Yamaha is right behind them with perhaps a bit more quality in their add-on features. Pioneer gives a solid product for the money, while Onkyo gives lots of add-ons but falls short of almost everyone else in build quality. It's great for the entry level home theater in a box crowd who is just starting out and has a really low budget... But, once you want to get serious about your A/V setup, then there are choices which deliver higher quality, and Denon is one of them.
