At their price points they are both good. I would probably give an edge to the Onkyo. I would also take a look at the older Harman Kardon AVR 325 or the new AVR 330. While they look from specs as not having as powerful an amp--they truly give all that power cleanly with all channels driven, which neither the Denon or Onkyo do.
I just hope your Polk speakers aren't of the 4 ohm variety--I hope they are 8 ohm. Much easier load to power. If they are 4 ohm I would definitely go for an HK unit or a NAD unit.
Overheating would be a problem for the Onkyo and probably the Denon at 4 ohm loads. This could cause considerable damage.