I went through a similar analysis recently. I had no particular loyalty or bias to any one platform, though I began to lean towards the XBox because of Forza 6, which is the type of game that most interested me. After looking at capabilities, I settled on building a PC for about $1000. That's a lot more money than either console, but you get way more power and an upgrade path that will likely keep up for the next five or more years. The ability of the games on the PC is just better than either console, and I can swap in a better GPU at any time to keep it up there.
Project Cars, DiRT Rally and Assetto Corsa tick the boxes regarding my interest in Forza 6, so not having that on the PC was taken out of the equation.
It does mean running Windows, which as a Mac guy for day to day work is a bit hard to get used to - so many things suck it's a joke, but as a game-launching platform it's adequate. Once in the game, the OS is irrelevant. Steam is fine as a storefront and launcher as well. There might come a time when having Linux as the launcher is also workable, but right now it lags behind in terms of graphics support (e.g. many games require DirectX11) so Windows is a necessary evil.