A few years ago it wasn't much of a contest who the king of sim racers was. Gran Turismo was bigger, better, and more of a car lover's wet dream than any other game on the market (super sims on PC aside, which are in a different category). I think at this point it's clear that Forza is the new king.
We can talk about how their physics model has gotten so much better in the most recent iterations of the game. We can talk about the partnerships with car enthusiast properties like Top Gear. We can talk about the passion they always show in what they're doing. Hell, we can talk about how they actually have the ability to ship a game, something the Gran Turismo team has proven to be criminally slow in accomplishing.
I've always been on team Gran Turismo, but it's become a team that I find harder and harder to fully defend. Forza is getting better faster than Gran Turismo, and Gran Turismo 7 (which presumably is coming out in 2020 or something) will need to do something really special to catch up.