I'm bias towards AMD too, I like the one in my MBP (see sig) I had a 4850 in my PC which was a little underpowered but no real issues, and now I have a 6970 in my PC and it ABSOLUTELY FLYS, also I think that the AMD product line up is a little easier to follow (but that's just me)
Well i'm leaning more towards the amd, especially with the fiasco I went through with my Santa Rosa MBP 17"... I had to get the nvidia replaced twice... fortunately the first time was whilst still under warranty and the second time was a freebie as NVidia finally admitted it was their manufacturing of the grfx card that was at fault....
The answer to this question shouldn't be based on preference or brand loyality. There are some reasons that speak for one or the other.
It can change each generation but this time I think Kepler (640-660M) is the best notebook GPU.
AMD clearly was more power efficient with its VLIW4,VLIW5 design. The last two generations always were clearly the better solution in a notebook. The new GCN architecture is a little worse in power efficiency which is offset by the new process but still a 28nm VLIW4 for gaming would probably get a bit more speed per Watt.
Nvidia Fermi was bad in power consumption. It was good for GPGPU stuff but not needed too much power. The new 28nm Kepler is a whole new thing and including the new process they claim a 100% increase in power efficiency. Since a new process usually only gets one 30-50% more power efficiency the rest should be architecture. The 680GTX also seems a bit ahead of the 7970 in the desktop while the 580 GTX was quite poor compared to a 6970. Thus I would suspect that Nvidia not only closed the gap in this generations but lead now.
On pure speed for the heat a 650M is probably the best bet with the new generation. There is not much known about the 7750M and if it was nobody really knows the true TDP in power efficiency because they don't tell. You need to check out notebooks and try over clocking to see that.
Generally if Nvidia and AMD would be equal, I'd still go Nvidia because they deliver the better drivers most of the time. For gaming that is worth it.
On OSX I think it really doesn't matter and I have no clue as I don't play games on OSX. Civ 5 is unplayable on a 330M that is all I know.
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.