Your definately gonna get more GPU power out of the dual amd cards. but unless you need that power, your gonna want the nvidia. amd cards have better hardware and can power through extremely tough loads. But amd is known notoriously for tearing and dropped frames. you will see higher average frame rates on the nvidia as well as cooler temps and no hassle with the crossfire config.
Things that need to be clarified for a proper solution:
what resolutions?
how many monitors?
will you be gaming across multiple monitors?
how picky are you?
if your only using one monitor, Nvidia all the way.
if your gaming one monitor at over 1080p, you may still want nvidia
when you push the resolution to an extreme however, the dual amds have more raw power and will yield better results.