I would say first and foremost, price, followed by performance.
But kind of both. Power third.
I've bought many different GPUs, from both AMD/ATI and NV.
6870s, which I used in my BIL's PC, to GTX460s, that I used in my AM3+ PC with a Thuban. (Tried SLI for the first time, it ran, but it sucked. Made the top card way too hot, had stability and compatibility issues. Don't plan on ever doing that again.)
Sold off / gave away my GTX460 cards to friends, eventually.
Bought some 7950 3GB HIS cards at Newegg last year, or maybe the year before, on firesale for $130 ea, no rebate. (I considered that quite a good deal, as the R9 280 was selling for $180-200 at the same time.)
Bought some 270X, 260X, 250X, on clearance from BestBuy through ebay. (XFX clearance stock.)
Most recently, bought three MSI GTX950 cards for $120 ea, from Newegg through ebay.
Was toying with swapping the 7950 3GB cards for GTX950 2GB cards, to save power in the warmer months, but I think that's step back for gaming, for the most part. But I would gain H.265 / HEVC decoding, and HDMI2.0. Which, if I get a 4K monitor, might be a win. (7950 also has DisplayPort, and can do 4K too, just not 4K@60 over HDMI.)
Also bought like 7 GT630 Zotac 1GB cards (Kepler, 384CC), because they were $35 ea. Works good for "just need a display" purposes, and watching videos, and HDMI output. The same exact cards, with a GT730 label on them, are like $60-70!
So, maybe I'm all over the place for my GPU purchased, but I love to snag a deal.