I'd definitely go with the Yamaha. Sound quality , easy controls, and reputation are my main reasons. The sound processesing hardware that Yamaha uses is on par with most middle to low end Harmon Kardon amps. Plus they're gorgeous. However, Onkyo's aren't anything to sneeze at, and at least they're better than Sony's. Well, I'm done ranting.
You ever look at the Yamaha logo? it's a triangle formed by 3 tuning forks, the univeral symbol of precision. Just a random useless fact.
Karl