<3 Top Gear.
Of course GT is going to have a larger following, it's set in a much older foundation. I, for one, grew up on GT and loved every minute of it. Forza 1 seemed very foreign to me when I first tried it and I saw vast improvement in Forza 2. I expect Forza 3 to be able to compete with GT5 but I'm not quite sure how it will pan out. GT has a few things going against it as well as the high expectation of a quality game. The PS3s have not sold well and I bet you Sony is praying for a huge upsale on GT packages whenever GT5 releases.
I do agree though, if you have a PS3 wait for GT5 and if you have a 360 Forza 3 is the way to go. I really don't see much sense in continuing an argument over games that are yet to be released.