HPX500 vs XF300/305
 

Anyone have a chance to use these two cams? Which would you prefer?

apples and oranges...I don't really see these two being compared as they are so completely different.
The only thing you probably could compare is the final picture quality.

I cant really speak for the Panny picture quality but based on the specs, I have no doubt it's great. I've used all the panny cameras from the 200 down to the hmc40.

As for the XF, every time I use it I know I made the right decision for the money spent. I can't really find anything to complain about other than the on/off switch. I prefer the smaller form factor and attach part of my dslr rig if I need to shoulder it. The image stabilization and face detection AF mode are just awesome. Using CF cards is a major plus to me as they are much easier on the wallet than P2. I load mine with 2 top shelf San Disk 32gb cards and I have a 3 more 32gb and a few 16gb for the price of a couple of big P2's. Then there's the fact that the CF cards are easy to transfer to any computer with a simple card reader. The only advantage of P2 to me is with FCP there's no need to transcode. Though since I can edit natively with the Canon's MXF files, not that big a difference.
Picture-wise, it's got an edge over the EX1r which I think it was aimed at. And it blows the lens off the hvx200 which I have a lot of time on. Seems the 500 is several notches above the 200 so as I mentioned, that might be a fair comparo pic-wise. But since everything else is so different, I think your first decision would have to be form factor. And then that other minor difference, price. You could get a XF300 and a XF100 for the price of the 500. And that is a deal I'd take but only cause I prefer smaller cameras.

I've edited HPX500 material and seen some raw XF300 fotage. The XF is sharper. The HPX has more dynamic range and a more pleasing picture. I wouldn't use the HPX for 1080p broadcast. In my opinion the 720p is the sweet spot for the HPX.