These threads always make me cringe.
One aspect that is never brought up though, is the community support that FM3 lends itself to. The customisation of the game is fantastic. The storefront is one of the best additions to a game like this and some of the work that can be created in the livery editor is sublime.
Also, the online community is fantastic (apart from T10 messing with the hoppers - but that's another story).
For instance. I'm on the Forza UK site which organises many competitions and racing leagues. I've just signed up to the Yaris Cup, which has been organised by one of the site's founders. We have 40 racers signed up and qualified, split over 5 lobbies (wish there were 16 per race as in GT).
FM3 has options so you can set up qualifying sessions easily (10 seconds between each car launch, sorts the valid times out, etc).
The forum is buzzing about the first upcoming race on Sunday and everyone is up for it.
We've all been practising together, pushing each other and having a laugh at the same time. It all gets serious on Sunday (well - sort of

)
Each racer has created their own livery for the event too (mine is below

) T
his helps bring individuality to the event in which all the cars are the same
What I'm trying to say is that it is not enough to have a decent offline mode these days as the community aspect of these games have taken over. It has certainly increased the longevity of FM2 and 3.
I truly hope that GT has a livery editor, decent online mode and community support so the likes of these racing competitions can thrive.
So, it's not about which game has the best damage model, the better interior or superior sound.
It's about having fun with like-minded people, making friends and having fun. THAT'S what racing games of 2010 should be about!