Depends on a few things:
-The spec of your pc. If you don't have a good spec PC it may be better to get it for PS3, but generally speaking the PC version is better if you can run it.
-Do you own a wheel? The PC version will be poorly optimised with anything but a wheel or an Xbox 360 controller (I'm assuming you don't have one of these). The PS3 version however, will be playable with a DS3
-Do you intend to play online in this game, if so do you know people with it on PC or PS3?
I got the game for PC (because it was like £15 cheaper during the steam sale than the PS3 version), but the majority of online events here on GTP are on the PS3, so if you are intending to race with people here go with the PS3.