Had some time in Project Cars and, if this is your game of choice, it's a no brainier, the Oculus Rift runs it significantly better.
For the Rift:
The screen door effect is less obvious, the world scale is spot on at default 1.00, and the brightness feel "nicer" in this game. In Elite, the Vive can literally blind you looking into stars, which is awesome, and something I'll compare to the Rift at a later time.
However, the biggest Rift benefit is performance and usability out of the box. The Vive needs config file tweaks, scaling tweaks, and constant seating position resets. Even then, the game can stagger along on medium to high settings on a water cooled 1080. With the Rift, I just whacked everything up to max and dialled back the shadows and grass a little and it runs smoothly. Brands Hatch Indy circuit in an old school Lotus in a thunderstorm at night looks incredible!
Against the Rift:
I've started to notice things are less sharp in the Rift. Sure, it has it's slight smeary and God Ray issues when you move your head around and look outside the central focus, but when you look really closely at dials, they don't come into pin sharp focus as well. I'm not sure if this is the anti aliasing, but something I've also noticed in Elite when looking at the jump drive countdown closely. It's minor though and maybe related to having the Vive supersampling set high. The Vive is definitely sharper with almost zero screen door when text is right in your face.
Obviously I can't do a back to back comparison as the Vive is in for its second repair, but it's easy to tell the differences in performance in this game. It will be Rift only from now.
Right now I feel justified in adding the Rift to my setup, but wouldn't be without the Vive for non cockpit experiences. Maybe that will change when Oculus Touch launches, but right now, having both headsets is the best option if you play a wide variety of games. PCars is still an awesome experience in the Vive (with the right setup), but it is even better in the Rift.