I don't understand why there are five of the same car in the game and the liveries and tuning set-ups are not interchangeable. Crazy. The Mercedes is much easier because there is only one, and there was only one Audi in Forza 3 so I don't understand Turn 10's thinking here. Car list padding?
I agree that if you have a PS3 then you can't miss GT5 and I'd imagine it's dirt cheap now.
I've tried to get back into it but just can't. I don't totally agree with your appraisal. The physics are better in some ways. I like how it's easier to get out of shape under braking if your car is unbalanced, I really like the track-edge grip reduction which can spin a car if you put a wheel on the grass (Forza needs this), and the game models things like lift-off oversteer much better.
I don't like the braking model nor the brake tuning settings. It's easy to drive Forza without ABS, but unrealistically hard in GT5. You have to use ABS to be competitive and I don't like using assists. I don't like having to use a 900 degree steering model in a racing game (however this can be overcome with a Fanatec). I'm not keen on the slip-stream effect which is exaggerated.
Graphically it's on another level to Forza when you get premium cars on premium tracks but distinctly PS2 in a standard car on a standard track. I hate the 800 standard cars with PS2 textures and cardboard cut-out cockpits, which reduces the usable car list to 200 for me. I hate the standard vs premium concept in general, both the cars and tracks, and the frame-rate drops during weather or night racing, or on a crowded track. Forza is more consistent and smooth graphically. Not track or car looks any worse or better than any other and the 60fps frame rate keeps it smooth.
Tracks - GT5 has Spa, Monza and Monaco, but overall it has less real-world tracks than Forza, and the likes of Silverstone, Catalunya, Mugello, Road America and Infineon tip the track list in FM4's favour personally.
GT5 has no livery editor and less advanced customisation and tuning options, which is a big deal to me.
Neither game is perfect in the end. GT5 does have a solid online mode though, with some better lobby options whereas Forza is atrocious. I genuinely didn't expect that to be the case.