I have a pair of overclocked undervolted powercolor 7870s that I got pretty cheap (at the time), and I love them, they run quiet and cool. Not sure how the longevity is gonna be, had my first powercolor card for just over a year now with no hitches. Got the second one last month. I usually show the love on XFX for that lifetime warranty. But I gotta say, I like the powercolor, pleanty of sensor information through GPUZ, Easily customizable. Mine came witha 2 year warranty, so we'll see how they fare in the future, but for the time being, recommended.
Do the math, I bet you will see that the % you pay extra doesn't even come close to the % increase you get performance wise. Remember that a 270X is just a 270 with a bit higher clock rates. Nothing fancy.