Forza 6 is twice the game Forza 5 is, so if you liked that you'll love this.
PCars is worth buying, but only if it's sub-£20.
Cheers Ekona, will probably go with Forza 6. I think I paid £22 for Project cars, but like I said, took it back after a friend said it was better on the PC. You've not noticed any bugs with it on Xbox One then?