Since this is the vs thread... here's a lil something of reality vs FM4 vs GT5 standard (no premium model available).
Very old shot of my real car:
Audi B6 A4 S-line, modified = visually it's from Stasis coilovers, & Audi OEM BBS CH/CO wheels
Audi B6 S4 - FM4 = mods include race suspension & BBS wheels
Audi B6 S4 - FM3 = Only 'side' shot I have right now, mods include Oettinger kit, suspension and BBS wheels
Audi B6 S4 - FM2 = mods include Oettinger kit, race suspension and BBS wheels.
Audi B6 S4 - GT5 standard = mods include race suspension and BBS wheels
Had there been a premium model, I'd post it up but this car is only in standard form. Forza 2's model looks better than GT5's version, so clearly Forza 3 and Forza 4 would look better by default. Had there been a premium, there's no doubt that GT5's version would be the best modeled version and best looking one in comparison.
Sound wise, I know what the S4's 4.2L V8 sounds like in stock and modified form, but my car is a modified 1.8T (1.8L 4cyl turbo). In GT5, sounds nothing like it. In FM4, sounds a lot closer to real life but not dead nuts to be honest. Sadly the S4 weighs more than my A4, is heavier on the nose and I've never driven one hard in real life. I can probably get a closer build in GT5 to my real car due to the power restrictions and weight bias adjustments.