Indeed, the gpu on the xbox360 is considerably more powerful than the gpu in the ps3, as far as I understood it, sony went with it this way since they figured that making specialised cpu cores to take over from the gpu would get them more performance and better frame rate, and indeed when things can be done directly in the cpu hardware things get real fast.
The thing about specialised cpu cores are that they are insanely more difficult to code for than generalised ones, like the cores the xbox360 has. In this industry I think we can say that
difficulty = added project development time. And development time is insanely expensive these days, So if publishers like EA (which sadly enough) is driving a large part of the games industry gets the option of making a game for the xbox360, which they can have done in a year or two, or making a game for the PS3 and spend maybe two to three years on the same project, its becomes easier to convince shareholders that launching on the xobx and reaping the profits from that sale while porting to the PS3 is the better thing to do.
On top of this the Cell processor is in it's infancy where the powerpc cpu has been around the block for quite some time, this meaning that compilers and programmers are used to working with this cpu, wouldn't suprise me if the early compilers for the cell processor still has some baby teething issues complicating matters further.
Again these are my own litte speculations, so please correct any potential wrongness, it's one of the ways I learn
Yes, I abuse grammar by opening my mouth.