People claiming either game is more relaistic handling wise are playing the PR game. Both games are very good simulations of the real experience, both also make some concessions but neither would be considered above the other from a neutral viewpoint driving simulation wise.
Forza is not a arcade simulation in the slightest although Forza 2 did have a arcade mode which basically gave you a car and track to go at alongside the comprehensive career. From a customization standpoint Forza allows you to change most components in the car (brakes, turbos, superchargers, wheels, bodykits/spoilers/splitters, suspension, remove interior, slick/race/road tyres, etc). On top of this you can tune the suspension setup, brake bias, etc. All in all the options are huge but to give you some idea of the depth check out this section on the Forza boards:
http://forums.forzamotorsport.net/forums/post/2164529.aspx
If GT5 is delayed until next year then I'd consider Forza 3, if both arrive this year then perhaps rent them and see which you prefer.







