The difference between ATI and Nvidia in truth is no different then a 1000HP/900ibs 4cyl and a 1000hp/900ibs V8.
Both are at the same HP/torq, they just get the power in different ways. It is totally personal preference, game coding that can support one better then another, and system specs.
In honesty I was going to give you examples of each, if I find out what specs your motherboard requires.