ATI:
Pros:
Lower power
Lower energy
Overclock well
Displayport
Single-GPU triple monitor
Cons:
Not as powerful as Fermi
6000 series is expected soon
Nvidia:
Pros:
Better price/performance if you buy smartly
Better DX11 performance
Better minimums
Better 2560/AA
CUDA/GPGPU/Physx
Cons:
High energy
Run very hot
Don't overclock as well
Have to purchase on a deal to get a good value
Sli needed for triple monitor
I probably missed a few, but that's the general gist.
ATI gives you a fluffy puppy
Nvidia runs over it