Wait, so Ferrari Challenge, which was released several months after GT5
is not considered a "recent car game"? And even though GRiD is not a sim, its a "car game" and that came out after GT5
as well.
The Forza and GT releases have always overlapped each other, it won't change. People with any sense will buy (or at least try) both games, remember, competition isn't a bad thing - monopolies are.
There are many reasons why you would choose Forza over GT, both have their strengths and weaknesses, as do all driving games ever made, they all are never perfect and they all have different strengths to them. Its why its a relatively easy market to break into if you manage to get a few key compenents right in your game.
In other words, the reasons for buying either game are slightly different and because they don't normally release close to each other, they don't have to worry too much about what the other side is doing. Though I will say that GT is ever so slightly winning the battle overall, though it lacks certain features, its overall experience is better, but even so, I wouldn't expect fans of either series to jump to the rival series regardless how good GT5 is.
Not that I'm saying GT5 should be rushed and Polyphony should ignore the competition completely, just that its not true that Forza is immediately inferior to GT or that the fans need "winning" over.