It really comes down to preference.
For me, I am a huge AMD fan and liked ATi better ever since I put a ATi 9200 SE into my desktop way back in I think 98.
AMD graphics have beat nVidia in power per watt and power per dollar (two of the most important stats of a graphics card) since the 3xxx series came out.
However, this generation I believe nVidia basically has it. Kepler cores have made a world of difference and I wouldn't be surprised if Apple put nVidia with Kepler into their MacBook Pros. nVidia has the better product this generation, pure and simple.