It depends on your hardware.
If you have a PC that can run Psychonauts on its maximum settings, it will look far better than the PlayStation 2 version. This is because the PlayStation 2 uses hardware roughly equivalent to a 2002 PC, thus meaning that a lot of the special effects in Psychonauts had to be removed. In addition, most of the character models and such have a slightly lesser level of detail.
The only point of contention might be the controller, as many people prefer to play platform games with a game pad rather than a keyboard/mouse. While you can buy excellent PC game pads (Logitech
do a great range), many people don't have them so it's extra money. If you really can't stand playing with a keyboard/mouse and don't want to buy a game pad, you might prefer the PS2 version.
The PC version is the most technically superior of all the versions, by which I mean it has the best graphics. However, if your PC isn't very powerful or you don't have a game pad you might not enjoy it as much.
Now you have the information, it's totally your call.