First of all, I have not auditioned the Panasonic XR55, nor the XR57. I can tell you that we are VERY happy with our new Onkyo TX-SR505 -- it meets all our needs, and then some.
I was in the same boat as you just last week. Here's what I did:
1) Make a list of features that you MUST have. These are the deal-breakers (if it doesn't do it, scratch that receiver off the list).
2) Make a list of features that you would LIKE, and prioritize them as best you can. This will help you narrow your list of candidates down to a "short-list".
3) When you're down to a short-list of 2-4 receivers (based on the above, as well as price, etc.), find their respective manuals online and open them all up, and compare them feature-for-feature ("side by side").
This is what I did, and because of my requirements and "wants", ended up with the 505 over the XR55 and 57 (and the others that made my short-list).