I don't like the subscription part of iRacing, but I absolutely loved the realism of the physics when I tried it at a friend's place. I don't have a good pc to take advantage of pc sims atm, otherwise gt6 would never get played lol.
Forza 5 seems to have a few very nice F1 cars, I'm envious of that and the livery editor they have over there, but I can't justify buying an xbone just for Forza, when it seems all cross platform games, including Project CARS (my most anticipated game of the current gen so far), will be better on ps4. If Turn10 brought out Forza games on ps4 I would buy them in a heartbeat. IMO Forza started off not as good as Gran Turismo, but currently they are way ahead. Obviously gt6 has things I like which fm5 doesn't, but fm5 also has things I like that gt6 doesn't. I just think Forza is currently ahead in the physics department, and while it doesn't have as many cars, the ones it has are the ones people want, whereas gt6 has some of the cars people want, and about 700 that most people don't care too much about lol.