Honestly you're either a savant at Forza 5 or just lying, the game is not forgiving at the edge of traction in just about any car.
Well really that all depends on a persons natural and learned ability within a specified video game environment, and how good their individual reactions are. Ive had people tell me they find FM5 far too grippy, and others have told me its far too easy to slide around the track. For me though I dont find either of those right. Ive played all the previous forza games with a pad, and FM4 with an MS FFB WRW (yeh, rubbish wheel I know lol). FM5 I have also played with a pad, but I also have the TX. I find that the cars handle how I expect them too, they understeer when I enter a corner faster than the grip available on the front tires, oversteer when I exceed the grip available at the rear, and go around a corner without incident when I get it just right. There is a real emphasis in FM5 for throttle and brake control, and it seems to pay off to to take note of that.
One of the things coming from some people I found strange, was how they would say "I cant drive the Ferrari 458 (or X car) anymore, what has T10 done to it" (this is stock and not tuned), stating it was uncontrollable, and that it would spin out anytime they tried to turn, touch the throttle, or anything else. I tested the car out myself on Spa, and personally found it very easy to drive stock, and great fun when you want it to misbehave.
I cant compare FM5's physics to GT6, but I have played GT1/2/3/4/5, and I very much prefer Forza's physics to the GT games I have played, and always have. I dont know why, I cant elaborate on it, I just prefer Forza's physics. Perhaps its as simple as I prefer the Xbox controller, which I have always found more comfortable to use. Ive always been told that GT5 is amazing on a wheel, but never owned a PS3 compatible wheel. GT6 I am passing on, I am holding off on GT till the PS4 version; I want to see if PD will ditch all their current 3d assets and start from scratch like T10 have. On the pc I currently play Pcars, Assetto corsa, I racing (ect, list can get pretty long). I will play pretty much any racing game, whether a sim or arcade, as long as I find it fun. FM5 is definatly on par with I racing on a wheel (in my opinion), and the F458 handles almost identically in Assetto corsa (a bit nicer to drive in Assetto).