I don't think you know what you're talking about. Have you driven any of the cars in real life that are in both Forza and GT? Well I have, and I can confirm that GT's cars handle and feel much more like they're real life counterparts. All the FR cars in Forza just slide because they can and Supercars are the easiest things in the world to drive. They aren't.
Firstly you said that you can confirm that GT6 is more accurate, but you only said that you've driven cars in both games. That doesn't confirm much by itself. I also have to ask which Forza you've played. Horizon is very different from FM, it drives more like FM3 than 4 or 5, and that seems to be intentional.
I haven't compared real cars to GT6/Forza counterparts, I've only done comparisons of each games physics engines informally. I don't consider either good enough to even compare to a specific car, and I think a good test of the physics would focus on general physics behavior rather than seeing if car A acts exactly like car A.
Things I've noticed:
GT
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overly simple tire model: this has a huge effect on physics and car accuracy
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dead suspension: weight transfer is barely present, combined with the tires it makes for an on rails driving feel
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narrow range of attitudes for valid aerodynamics coefficients: Your rear wing will still work fine and cause understeer even if you're basically sideways. GT may not consider pitch/roll/yaw at all when it comes to aero forces
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point body behavior: The car sometimes feels like it's a single, compact mass, especially noticeable when accelerating from a standstill (torque steer) or braking hard.
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Excellent force feedback: This is really all I can give GT. The information through the wheel is very helpful
Forza
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Tire model: More diverse behavior across pretty much all driving conditions, and the grip level is not hardcoded for each tire (they may be doing something like GT yet, it's just more elaborate)
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Clear suspension model: Weight transfer is far more noticeable, though I think it may still be dumbed down to a degree as you can make drastic changes to suspension and only suffer moderately. Suspension tuning in GT5 felt like it did nothing. I have not been able to try GT6 tuning.
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Aero: It doesn't feel like GT's, when the car is in an odd position, I don't feel the aero. Hard to say things specifically since aero data is hard to measure from either game.
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Full car model: The car feels like a car. I felt like adding this point mostly because of my experience with brakes in FM5. I actually didn't feel as if I could just hold brakes at maximum and take my hands off the controller, the grip on the front tires was not always even. FM4 and all GT felt pretty dull on the brakes.
As for FR cars sliding in Forza, I can't relate. Definitely a non issue in FM4. GT5 on the other hand seems to have some really odd issue with grip for specific cars. The Speed 12 and Saleen S7 are my favorite examples of GT's buttered tire cars. The rear wheels simple can't take any force. I can't think of any similar example in Forza. FM4 had cars that just had low traction (like the Z4 GT3 car and SLK55 AMG), but they didn't feel like they were on ice. FM5 has made the edge of grip more dangerous and I did find the cars harder to control, but I think part of that was down to very bad non linear steering put into the controller. The 2013 Viper required a mild amount of attention to drive. The Ferrari F12 I could drive with one hand. The Audi R18 was disappointingly dull, much more fun in FM4 where it could more easily overpower the rear tires. The Indy cars were great as they had enough power to send the cars off the track and you needed to pay attention with them, but even in that case understeer was more prevalent than oversteer.
Another great difference between GT and Forza comes from muscle cars. Generally heavy and soft, they were pretty floaty in Forza while in GT they felt exactly like modern cars but with more understeer. It's like GT has a generic chassis for everything. When it comes to race cars I'm sure that's partially true, the race cars in GT often won't have unique settings in the tuning screen (and in fact the range of suspension settings you get for something like a Land Rover were no different from a Miata). Forza had unique adjustments that seemed car dependent and the stock tire for each car was custom defined. GT doesn't even have a non generic tire.
And subpar physics engine? Are you being serious right now? Compared to 5 and Forza, the physics engine alone makes GT6 worth buying.
I'll say right now I don't think GT can directly compete with Forza when it comes to physics at the moment. 6 may be better than 5, but I didn't feel it. 6 kept the on rails feeling when I tried it. I do know that PD has made suspension improvements. That was felt in the GTA demo, but these improvements don't even let them catch up to FM4 in my experience. If you want to compare GT5 and GT6, just go back up to my list of GT features and maybe cross out the dead suspension.
Cars have actual body roll now and tuning actually works for your own setup like say a real car.
I can't speak for GT6 tuning. Body roll is better yes, but that doesn't automatically mean the physics engine is right.
And what's not realistic about an accurate star map and a lighting engine that actually has some people confuse the game for reality? Say whatever you want, but I'd play GT in a heartbeat.
Those are nice things, but they don't have anything to do with driving. I don't mind your preference, for some people GT is a better game. I just don't think it's a better simulator.
It's completely illogical.
Huayra '11 - 613 PP
SLS AMG GT3 '11 - 610 PP <- it's a god damn race car.
There is no rule that says race car > road car, especially if the tires get left out. I'm pretty sure the Zonda will outaccelerate it at high speed.
Viper SRT ACR - 580 PP
Lister Storm V12 Race Car - 580 PP
Is the Lister still grossly overweight? The Viper is making 5-10 times normal steer car downforce, and remember tires don't get factored in. The weird thing here though, in GT the Viper barely makes downforce.
Corvette ZR1 '09 - 574 PP
Honda HSV-010 GT Base Model '12 - 573 PP
This one I can't really explain, unlike the Zonda/SLS example, the HSV has a huge weight advantage.
How does that even work? It's not usual that I see road cars pulling times similar to race cars from their PP range.
It works because that's the formula. For a PP system to be functional, you need to mathematically capture the impact of a car design on track performance. This has its challenges and limitations. Specifically on limitations, a car's performance ranking will change with track. If you only have one PP number per car, there is a bit of arbitrariness. PD should give us more options to use along with PP to make cars even. If PP can be improved they should do that as well, but I think that's more long term than short.
