Both games offer lots for the money are more or less multiple games in one. GT even tried to incorporate F1 and Rally, more or less successful although. I play most games in other genres about 20 hours in average. But i have been playing Gt5 since almost a year now and Forza 3 about the same time before that. So Gt4 was replaced by Forza 3 , which eventually got replaced by GT5.
If you keep enjoying a game for a long time, there must be something you like to do repeatedly. In Gt4, GT5 it was hotlaping the nordschleife for me, in Forza 3 it was the great online multiplayer part. But i didn't have so much time to play online anymore, so i cancelled my x-box gold membership. Just didn't seem to be worth the money as i didn't have so much time to play. I actually believe it should be free. I paid for the game , i paid for all the dlcs so why pay just to race other people? World of warcraft seemed to offer more for the buck.
Anyway i stopped to play Forza 3 because i had to pay for what i enjoyed most. I couldn't lap Nordschleife because it was just not accurate and didn't give me the results i expected. ( I drove my real life car, the crossfire SRT6 on Nordschleife. Accoring to car magazines it should be good for a time about 8:30. All I managed was 9:00, although i had experience with the track from Gt4, although i was faster than 98% of the leaderboard on this track. That was quite depressing. After i got some information in this forum , i know the track is too long.) Anyway I expected to replace GT5 with Forza 4, because a year has passed and GT5 has done its job. But now Turn 10 is selling me the same Nordschleife they have sold me 2 years before and had been depressing me. So did they hit the nail for me? No, they didn't. If they wouldn't charge money for their multiplayer, i would play Forza 4 longer for sure. But I guess I will get the gold membership for a few months , get some Dlc and then put the game to rest again and cancel the membership so i can spend the money on something else.