Long post warning:
I'd go for an Xbox One if I was just looking at racing games and didn't own a PC with an equivalent graphics card to the Xbox one, err, one.
On that you're getting Forza Horizon 2/3 and Forza Motorsport 5-6, possibly even 7. So that's four or five of arguably the best simcade and console sim racers on one console. Plus I think the controller and UI are still a bit better on the Xbox One.
Also there are slightly better value bundles for the Xbox One and if you're in the UK all Amazon bundles right now come with a free Forza 5 'Game of the year edition' download.
Right now on the PS4 you've got the arcade racer 'Drive Club' that's like 'Horizon' but not as good.
GT7, from the interviews I've read, sounds like it's going to keep every problem from GT6 although maybe they will fix playability issues like controller throttle/steering mapping. I think the only real improvement over GT6 will be the track visuals and that it will run at a solid 60fps in full 1080p resolution not three quarters of it.
GT6 is still selling well because it has no real competition but many people must enjoy playing it or it wouldn't sell so well. I'm not one of them!
FM6 I have complete confidence in. On the 360 FM2 felt slightly disappointing to me, like a higher resolution FM1 but with far less cars and tracks. I think they fixed ever major criticism of FM2 with FM3 same as they will do with FM6.