I would say PS3 since its the newest and Sony did it so to take out the 360. It's gonna smoke the 360 in performance and visuals with it's RSX engine run with a 8-core cell processor. Basically all the newest consoles outperforms the older ones and it's been like that. So I have a lot of hopes that the PS3 will perform just like the specs states. Have you seen them demo from PS3? Real time graphics looks like it's prerendered, yes, it's that good of the PS3 visual.
The 360 will have better graphics than the PS3. It comes down to the GFX cores. RSX is not a bad GPU. Its just a underclocked 7900gtx; same exact specs except for core/mem clock. The 360's Xenos GPU is based off of the R600's archechture, ATi's next-gen GPU due in Q3/4 of this year. While Xenos may not be as powerful as the upcoming R600, but when factoring in the unified shader archecture it appears that the Xenos is a good match to 1900xtx, the current GPU champ. Beyond that the Xenos has an advantage over RSX qualitatively, as well as quantitatively. Since it is based off or R600 it is DirectX 10 compliant, and uses the majority of the DirectX 10 API. It also has the bonus of 10 MBs of "smart EDRAM". The EDRAM daughter chip on the GPU does all AA calcs before output, giving it the ability to do 4x AA at less than a 3% FPS penalty. Add in the fact that no nVidia 7x series GPU can do AA and HDR at the same time (huge benefit for a game like Oblivion), and there is no evidence that the RSX could match Xenos. To make things even harder for RSX, Sony has announced that RSX will have to do all the audio processing for PS3 as well. As for Cell, while the potential for the CPU is tremendous (if you can keep all PPE's filled with data to crunch), a system is only as good as your weakest component. You could have a 20 core Cell if you want, but something has to actually draw the images on the screen and RSX simply wont outperform the competition.