I am partial to Yamaha as I have owned the Yamaha RX-Z9 for a long, long while now, but neither of these is more than 1% different, so I don't think you can go wrong either way. Here are some of the differing points that might make me lean toward the Yamaha:
Cost: $370 vs $500 - Onkyo (so if $130 is worth more than the below added features....)
Phono: I still have a great turntable and some old, irreplaceable vinyl; the Onkyo has no phono input.
Component Video Bandwidth: 60MHz vs 50MHz Yamaha is 10MHz higher
Preamp Outputs: 10 vs 3 - Yamaha (if you ever want to up the power).
Main Speaker Output Pairs: 2 vs 1 - Yamaha (multi-zone) I love this feature, I run #2 in-attic out to my deck to listen while we BBQ.
THD: 0.08% vs 0.06% - Yamaha (possible negligible amount)