You see, this is a trick comparision... :lol:
Me, as a Gran Turismo fan, will tell you that GT6 won't disappoint when it's about Gran Turismo series. If you know them, you know how they're. And FH2 is as said, an open world racing game...
Now, GT6 is for PS3, an outdated console, and FH2 is current generation... so, they look really different from each other, not only in the genre (circuit vs open roads), but also in graphics and those machine necessary things.
If you already own a PS3, GT6 definetly worth it... if you have enough money to get a Xbox One (Or already own one), FH2 definetly worth it aswell. As that is something i'd do if i had enough money.
Now, you only got money for one, and can't get both, i'd suggest the FH2. Because it's newer, and the console will have a longer useful time, compared to the PS3. Not to mention that GT7 will be for PS4, but it will be released near 2016, i believe. :lol:
And about the car list, well, GT6 has over 450 highly detailed vehicles, in total of 1200. Most of those 1200 are indeed japanese, which is something that i quite like.