The HD 6870 will be the better choice if you can afford it. They are usually near each other pricepoints after MIR anyway. Although I have no experience with XFX, I do know they have lifetime warranty BUT I can't say the same for the support you will get when you want to get a replacement [warranty]. The customer service could be non-existence. Gigabyte 6870 does have a triple fan design and that will help with cooling. It is a big graphic card so I hope it fits in your case. I think you should invest in a HD 6870 and then use it until you have enough money to make the leap to Keplar or AMD 7xxx when the time comes.
i should show you the specs of my computer im getting first
Intel core i5 2400
G.skill ripjaws 8gb 1600mhz
Coolermaster Storm enforcer case
Coolermaster GX 750W
XFX radeon 6850 or Gigabyte 6870 (not windforce)
Seagate Momentus XT 500gb