I've never played Forza....... but it must be one sad ass game if it's inferior to GT5.
OP, you may like GT5 if you are completely new to racing games/sims, if you have any experience with them and want a good racing experience... stay away from GT5. It does let you take pretty pictures and racing online against friends is great, but the in game racing against the AI cars is the worst possible, PD has no clue on how to make the AI even remotely playable and it ruins what otherwise could have been a great game (well it has other issues, but the AI is the one I hate most).
There are other good racing games for both consoles, try to test some on friends systems, or rent games to try out before you decide.
The Logitech G25/27 wheels are good entry level setups.