My understanding is that, technically, the PS3 has the capacity for better graphics, but that doesn't mean that developers are going to actually utilize that, and, its not like the 360 graphics are horrible.
I would definitely go with the 360, since you have the awesomest of all things, the Wii, already.
For one, it's cheaper!
Plus, more and more PS3 "exclusives" are being announced to be on the 360 also... because the PS3 is
not selling... they are losing more and more support, and there is no reason to buy a system that is going to end up dead by the end of this gaming cycle.
Now, its too early to say there is a clear winner in the new format war... if Blu-Ray wins out, then yeah, the fact that the PS3 can play those is nice... but from what I've heard the HD DVD is winning the war. Sony put all their chips in the Blu-Ray basket and as of now it looks like that is going to fail them.
I do however, offer another alternative... buy another Wii and send it to me!