Just thought Id add my 2 cents. From a bloke that owns and plays both Forza 3 and GT5, Forza 3 is the better 'pick up and play, introduce your mum to it' as such. Like if you got a mate round who doesn't really play racing games then Forza 3 is the best.
GT5 is more of a solo game for me that grows on you. At first you will think 'what's this all about' and you really need to invest time in the game to get the most from it. And if you are a serious driving enthusiast, a Logitech Force Feedback wheel (I saw one for 60 quid on google shopping) and GT5 are heaven.
I am not saying Forza 3 is easier or less 'real' than GT5, but it is definatley easier to get into and it is laid out in a more friendly fashion for those who aren't petrolheads or veterans in the GT series.
My real life example would be that I got GT5 for xmas and the disc hasn't left my PS3, because everytime I turn it on to play a game, I forget about all my other games when I see the GT5 disc pop out and I immediatley put in back in. I don't know why!
Saying all this, it is just opinion. And I am a massive fan of Gran Turismo. I love driving 1.6 Peugeots and stuff just as much as the ferraris, but the one thing that I feel everyone will agree with me on is that GT5 is not the kind of game you can rent out for 2 reasons:
1) You will not enjoy it as much as you would be able to unless u invest time and progress and
2) If you rent it you will incur a fortune in late charges cos you will not be able to remove it from your PS3

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