GPUs on all these platforms can be programmed through a set of extremely similar APIs and all share the same modus operandi when it comes down to vertex and pixel shading, therefore what makes a big difference here is the CPU and the amount of available memory (I'm not concerned that much about differences in GPU performance as GPU content is usually easier to scale).
It's clear that PS3 has the most exotic architecture compared to the other three platforms, and also a (relatively small) memory disadvantage. While the latter is a kind of bottleneck that can be easily overcome (again, scale the content) to other platforms detriment, the former is certainly not that easy to address, especially if you are in love with OOOE CPUs (and who's not?)
Programming everything from the ground up to be efficient on PS3 would benefit 360 a lot too, while PC and Mac platforms really wouldn't care that much. Again the choice you have to make is a no brainer here, PS3 will end up being your bottleneck in many cases, while CELL will mostly go unused. I guess Sony has learned a lot of lessons this round, and if they have not, well.. who wants to open a restaurant with me?
A last recommendation for fanboys, no matter what's your faith, don't count the number of SPUs or CPU cores used to find out which platform is more powerful/cool/etc. Just give a look on what you see on screen and draw your own conclusions.