Hey Anazarr! With Powercooler i don't know 100%, but from what i did read the RMA can be a nightmare, you should check it out/google but i personally would not from reading a bit into it. Also this is only in the situation of it going wrong. For £10 extra though i would probably go for gigabyte for piece of mind (i was looking at that same card a few weeks back when it was on offer) There are a few sites that have some RMA info for graphics cards. The ones that aren't so great usually take upto a month/6 weeks to rectify a problem, GIGABYTE as far as i know had a turn around of about a week, but ofc that is subject to change.
I'll have to try and check out KFA, although i've never been a big fan (arf arf) of reference type coolers. Anyways, i hope this helps man!
I can only offer advice, the decision is yours to make which ever way you choose, at least you've narrowed it down to a 270 or 660 though