Well.. Technically they both can produce the same graphics depending on the implementation of them. You could have a game that's 1 room big and has amazing graphics or a game that's 1000 rooms that has crappy graphics.
I would say as far as capability goes, I would put my money on PS3 if only for the blu-ray support it has. The current standard for DVD-R's is ~7GB while I believe a blu-ray disk can be ~24GB. So, PS3 has more potential to store more gameplay in graphics, but the official specs were not released. http://wiiudaily.com/wii-u-system-specs/ If this source is correct however, the wii-u wins in performance.