As FT-1 mentioned, some people find it hard to justify any console purchase for a single title. I got my XB1 largely for Forza, but it wasn't until the run up to FM6 that I got it, so I could enjoy FM5 and FH2 on it beforehand. I also grabbed PCARS for the system, and a friend gifted me Need For Speed.
To be honest, it would've been worth it in my eyes even if I stuck with just the Forza franchise, as I've gotten a lot of play out of the games. I use the One more regularly than my PS4 for actual gaming — the PS4 is our Netflix machine of choice — so on that basis I can't complain.
In terms of racing games, I'd say the One is currently the pick of the console litter. The only exclusive PS4 has of note is Driveclub: it's very pretty, but feels small and hypercar-heavy in comparison to FH2.
Also, if you're planning on sticking with a controller, the Forza games are very well-optimized for them. I really appreciate the rumble-effect of the triggers; it helps when you're running without ABS. If you're looking at getting a wheel, the waters muddy a bit. There're great options for either PS4 or XB1, but nothing that covers both anymore (with Fanatec's Sony support recently being pulled or patched out in games).
Getting a refurb for cheap is a good move: that's more or less how I jumped into the 360. 👍
I'm curious, how active is online right now in FM6? Also, it it pretty easy to find some good clean racing, or is it full of rammers online?
It can get messy online in the public lobbies. Leagues are a bit better, but even then you still find people trying to win the race at the first corner. Finding individuals you can race with makes things massively more enjoyable.