Sound quality? Probably not.
However, the Onkyo has several features the Denon does not.
Just a couple off the top of my head.
- Dual HDMI outputs
- Bluetooth built in
- Network capability
- Wi-Fi
- Phono input for a turntable
- 7.2 (so on extra subwoofer output)
- Zone 2 Preouts
However the Denon does have one leg up in the auto setup. It features Audessey MultiEQ XT as apposed to the Onkyo's standard Audessey MultiEQ. So it could be argued that the Denon has the potential to sound better assuming the Audessey setup is used, but I'm willing to bet that we'd be splitting hairs.
A friend of mine has the Onkyo TX-NR626 and he loves it.