I never purchased Gran Turismo 6 due to how much I enjoyed Forza 4, after playing Forza 4 for 160 hours I purchased Xbox One and I like Forza 6 even more. Here is a chart I made explaining the pro's and cons of both games in a comparison and included features one game has that the other may lack in the Pros. To keep it fair I won't be comparing the graphics.
Gran Turismo 6
Pros
-More Cars
-Dynamic Time of day/Weather transition (this one may not matter to some people)
-Track Editor
-Rally Racing
-Cars are slightly/moderately more photo realistic depending on the vehicle
-44 Tracks, More Than Forza 6
Cons
-Bad Sound
-Bad AI
-Bad Car Selection (About 460-470) of 1247 are Premium
-Not as many cars on track
-Lack of Car Customization/No Player made paint designs
-Less Tuning Options
-Not as many multiplayer game modes/lack of multiplayer depth
-Lackluster Damage System
Forza 6
Pros
-Better Sound
-Better Car Selection (580-620 cars all rendered the same with interiors)
-Better AI
-More Tuning Options
-More Car Customization/player made paint designs
-More cars on the track
-More multiplayer game modes and multiplayer Leagues
-Better Damage/Crash Physics
-Forza Vista (Having this may not matter to some people)
-Better Sense of Speed
Cons
-No Track Editor
-No Dynamic time of day/weather transition
-No Rally racing
-28 Tracks, GT6 Has More
Gran Turismo 6 cons really bothered me which is why I never purchased it. I really hope Gran Turismo 7 can improve upon these major flaws. I never played the full game of Gran Turismo 6, only the demo so some of my points might be wrong like car customization and tuning options but I think standard cars have less tuning options available. I am also not 100% sure about the multiplayer in Gran Turismo 6.
Anyway, what game do you all prefer and why?