In your place, although I might have ordered the XFX in the first place, I don't think it is worth the hassle of changing.
I have bought cards from both brands. Both have been mostly good, UNTIL I got a XFX HD7970. It may have had a defective cooler, because it howled and ran hot. XFX Support did not get back to me in a timely manner about whether or not I could reseat it without voiding the warranty, so I sent it back for a Gigabyte WF3 model instead. Although that card is huge, its cooler works very well, and quietly.
I have never had to contact Powercolor support, although the cards I've bought from them have all been really low-end. I have seen a guy from their support, "Hugo" I believe, actually replying to customer comments on Newegg, although to be fair I think I've seen XFX there as well.
Edit: I wouldn't touch Diamond with a 10' cable. They're on my personal "Do Not Buy" list for a multitude of sins. Although its old news (mostly based on events concerning a HD3870), nothing leads me to believe I have anything to gain by buying a Diamond, vs. any number of their competitors.