If you take a look at
WhatToMine calculator it will give you the most profitable coin to mine for the GPU's you have, or, would like to get.
GPU selection would depend on the coin you want to mine, as, from my research, each GPU vendor (AMD/nVidia) has its own strengths.
I guess, when it comes to power, it depends on what you're willing to spend, more powerfull gpu's = more power draw, which could/would affect you profitability.
Have a look at the profitability calculators around and then, from there, make you decision on what hardware/coin is suited to your situation.