As far as sim racing, yes, I think GT is by far the better franchise (thus GT5 is the better sim). On the other hand, as a game [overall], that's where things start to become much more even on the playing field (with Forza actually topping GT in certain aspects).
And the same way how Killzone tops Halo in a lot of aspects. (Graphics, melee, better voice acting, more realisitc, weapons e.t.c.) but does that make it a better game than Halo? Halo has a much better story, more remembeale characters and mch better online e.t.c. So Forza 3 may be able to beat GT in one or two aspects but hardly any. I bought Forza 3 only because it got a 92 meta. People said it was something original and different. But it was just a fail copy GT 5 prolgue! Within a few days I threw the game in the trash (first game ever to that). The AI were dumb the online sucked, graphics weren't good at ALL! It felt more like GT made for 6 year old kids. To be honest Forza 3 is the worst game I hve ever bought. I was lucky enough not to buy the steering wheel or I would have got even more pissed.