As I said in the CB, they're both fantastic games at the end of the day.
Sure, they both have their positives and negatives, but they're just games. They're both very respectable games at that. If you have a PS3, GT5 is the best racing game to have. If you have a 360, FM4 is the best racing game to have. Depends which console you have and which takes your fancy. I've played both and I highly enjoy both games, Forza has much more of a 'fun' and community based appeal, whereas the quality and overal 'formal' feel to GT5 I love.
Personally, I don't do drifting so it doesn't matter to me about the drifting, but Forza's is very respectable as is GT5s. Also, the AI in FM4 appear to be much better and more unique, with them randomly spinning out or crashing occasionally, but GT5 is still a joy to race on.
So, they're both great.
/thread.