well, i have played both at the same time as my tv is right next to my comp. If you have a decent computer with a decent videocard (i have gf4 ti4200) I can say that it runs nicer and looks nicer on the pc. BUT if you have a kickass ps2 setup, amp, huge tv, nice big foff couches, go the ps2.
So yeah nfs:u dont run as well on the ps2 but its better value because of the arcadeness it has. Basically, what ever you get it on its still rice heaven.