The Wii version is being played on a TV which happens to be a far superior screen, and with a better controller in nearly all cases.
That is not to say that the PSP doesn't have it's advantages. Being able to understand and read things does help a lot. In most cases the PSP version would be the best, presuming that you have the TV hookup for your PSP. The bad part is, the PSP's TV functionality makes the screen smaller than the TV screen, for god knows what reason. However, I hear you can 'fix' that if you're savvy enough with your PSP.