The Yammie will accept audio over HDMI and can decode the hi-def formats. The Onkyo will only pass through the HDMI signal, so you have to use a separate connection for audio and can't use the hi-def codecs. However, Onkyos use Audessey, which some people prefer to the YPAO in the Yamahas. The Onkyo 507 is supposed to do this, making it more comparable to the 465.
The Onkyo is available from shoponkyo.com for ~$200 as a refurb. I got mine there and it works flawlessly. Free shipping too.