Can't decide what to pick for first mining rig 270 vs 280



I really cannot decide what to buy for my mining rig to start with.
 
280x has better kh/s but consumes more than the 270, but costs 120 $ more .
 
On the long term which one would benefit me more ?


With 280x i would get more coins but higher energy consumption, i pay aprox 15-18c for kw/h

270 is cheaper and lower energy consumption.