I can inform you on the Yamaha only, as I have no experience with the Onkyo.
I believe the 797 is the best 2 ch receiver you can get for under $600.
Here is what gives it the edge:
1. Lots of clean power - 100wpc with less than .03 THD. I am running 4x B&W 685's and a Sub on A+B and it produces powerful clean sound. Yamaha's power ratings must be much higher than advertised.
2. Variable Loudness contol - It is basically a midrange tone control that starts at zero and goes negative from there.
3. Pure Direct & CD Direct switches - comes in handy when you want to bypass the tone controls. (I use it when watching DVD's and on certain CD's)
4. The FM/AM receiver section is fantastic. I just left my antenna in the cabinet and it picks up local & distant stations better than any I've had.
5. Subwoofer Out - Well this works great if your sub has a crossover through it's Sub RCA input jack ( Mine doesn't). The 797 Sub out sends a full range signal. This is it's only drawback for me. Why have a sub out with no bass management or crossover? I have to run speaker wire to my sub to use its crossover.
I have had my 797 for 3 years now and am only slightly tempted to upgrade once in a while, but when I give it a good listen, the upgrade urge goes away. The 797 really compares to 2ch units such as the OUTLAW/NAD/ROTEL etc, but at a fraction of the price.
BTW, have you looked at the Harman Kardon 3490? This is also a quite respectable 2ch receiver at a low price point.