Those two are really a toss-up. The Denon has a new Anchor Bay video scaler (ABT1030) that is a somewhat unknown quantity at this point, but based on the performance of that chip's big brother, the ABT2010, (which is in your Oppo) it shouldn't give up much if anything in quality to the Faroudja solution in the Onkyo.
One clear edge the Denon has is its room EQ capacity. It has Audyssey MultEQ, which has twice the filter resolution of the 2EQ included in the Onkyo. 2EQ also has no subwoofer correction capability.
Either will do you fine, though.