Here's the deal:
If you have a ps4 and want a sim racer, get PCars, it runs well now. If you prefer an arcade racer get Driveclub.
If you have an xbox one and want a sim racer then it's a tough one. Technically the game is poor on that platform, but the core of the game still remains so I'm sure there's still some fun to be had. However if technical issues/bugs are important to you then get Forza 5 or wait for 6 but bear in mind that the core racing experience probably won't be as good since you can't have longer races with pit stops, fuel/tire wear, qualifying, practice and set custom race lengths. The weather/night aspects will also not be dynamic in FM6. So it's a bit of a toss up and depends on what kind of experience you want and how much importance you place on certain things. If you want an arcade racer on there get Forza Horizon 2.
If you have a capable gaming PC then ignore everything I said above and get PCars on that.