In terms of VR implementation? It's better (more complete) than AC at this stage, but considerably more demanding. If you're struggling to hold 90 in AC, you may want to hold off on PCARS until you have a faster system. That said, many people seem to be ok with playing PCARS (and AC) at a far lower framerate thanks to ATW. I can't stand it myself, but there you go. As mrklaw says, PCARS is less 'swimmy', most likely thanks to DS2X AA, which is a form of downsampling. You can achieve similar results in AC with the debug tool.
[edit] Now no longer needs debug tool, they've added an INI setting.