I'll have to add a couple of bits to the physics argument, having forgotten Devedander's feud.
What was my feud?
BTW I will chip in that I think a lot of this comes down to way more factors than just a physics engine... and a lot of it also comes down to what people think is better or more real.
Often people will think that something harder is more accurate which is not always the case.
That said I can definitely tell a difference in Forza feeling and GT feeling... to me:
Forza feels sort of smoothed and slick while GT feels very twitchy and harsh.
Forza feels like you are racing a dynamic set of circumstances and it's less a case of doing it just right to be perfect but doing it to be well overall while GT feels almost robotic and exacting in terms of how things unfold.
It really feels very much like a classic "American vs Japanese" take on things with American take being very open accessible in a lot of ways and the Japanese version being a case of almost a mathematical formula to get right or miss.
It feels to me like in Forza at any given time there is an area of the track and a general speed you want to get as close to as possible, but the dillinations of how far off you can be are almost infinite and as you get farther away the results get slightly worse and worse.
With GT I almost feel like it's the matrix where there is some solution and it can literally be hit perfectly and the steps of error are almost set in feelable size chunks such that it's a game of practice and repitition to nail something just right... almost like a rock band or something.
I have no idea which is more realistic as I don't race cars, and I dont' even have a favorite... both satisfy a different side of me. Forza satisfies the smooth feeling of a smooth driving experience in which I am just sort of doing what I feel is right at the time. It's almost a serene experience as much as racing high speed cars can be. GT satisifes the grinder in me that loves a "right" way to do everything and the rewarding feeling you get when you feel like you pressed just the right button combination at just the right times to trigger the perfect result. It feels like a game of practice and nail it perfectly like a music rythm game.
Outside of these the controllers have a lot to do with it. How the controls are mapped to the game, how much dampening is thrown in, how quickly steering recenters, how quickly momentum and body roll is recovered from. I often feel Forza is a little too stiff, especially after you have upgraded to the racing suspension... but then racing suspension should feel pretty stiff. Conversly GT often feels like the momentum and roll is exagerated and maybe even takes too long to recover from.
This leaves Forza feeling a bit harsh and predictable (but then a race car should be predictable as you tune it to get predictability) and GT feels a little mushy but that mushiness also heightens my sensation of being in danger and being on the edge (somethign that maybe is helping me make up for lost feedback from momentum that your body should be feeling in real life).
Now these words I am using to describe it are like trying to describe a temperature or a flavor... they are as close as I can get but don't try to take them too literally as they obviously will have intonations they shouldn't in them... for isntance I am not saying Forza feels like driving on ice all the time or that GT feels like it has bad steering. This is just me trying to isolate an explain what I feel the differences are and how they contribute to the experience.
At the end of the day there are two things I think we should be looking at:
1: How the game figures out what should be happening (ie the math behind it)
2: How the game makes you feel like things are happening
They could literally both have the same math behind the scenes (they don't but just saying) but if one of them chose to give a view as if the camera is bolted to the dash but the other choose to give the view as if the camera is glued to the drivers helment, you will get a very different feeling driving either.
Similarly control wise, how analog control sticks being transfered to a car steering system is mapped will make a huge difference in feeling of the game.
Even the buttons... I find after getting used to one controller, the other is trikcy to jump back into. For instance after using the PS3 shoulder buttons for gas and brake the 360 feels really weird and I find myself getting on the gas an brake way too hard for a while and vice versa.
So that's kind of off topic but just thought I would throw my 2 cents in.
Oh and has Forza suppressed my appetite for GT? Well sadly I haven't gotten it yet, only played the demo a bit (my 360 is packed, some ratehr serious business dealing with my house is preventing me a lot of free time) but I don't see how one great game can't help appease you while you wait for another...
