The argument comes from you trying to make what is a matter of opinion, a matter of fact. It is your opinion that Forza has better physics and is the more realistic driving simulator. You are certainly entitled to that opinion, but simply having that opinion does not automatically make it a fact that can no longer be argued.
Indeed, most reviews that I have read on the PAL version of GT5: Prologue disagree with you. As 1stnewcar said, at the end of the day it's all personal preference.
If you have access to a PS3, rent it when it comes out on the 15th and give it a chance. I'd love to hear all opinions before I drop ~$350 on the game and the G25 wheel...