he didn't even mention GT5 because he knows he can't beat a mammoth

its like GT5 is the "wii" & Forza 3 is the Xbox 360 & This NFS Shift game from EA is the "PS3"

While they are arguing amongst themselves, GT5 will overtake em both lol

But isn't by him saying that Shift > Forza 3, he is also saying Shift > GT5 since T10 say Forza 3 > GT5?

Only if you actually believe what Turn 10 said. 

Personally they only mentioned those games because GT5 wasnt playable at E3 or has a confirmed release date.  If it had both, none of these comments would have been made.

It also depends on what areas they think they are superior as none really say. GT5 will have the most cars and most "modes", Forza 3 will have the most customization and tuning, and potentially better looking vehicles (anyone's game though as Shift looks amazing and GT5 gameplay hasn't really been shown yet), Shift... uh, can't think of anything off the top of my head.

Well maybe EA thinks that Shift will be the better driver with better physics.  Which in the end is what is really important in a driving sim.  Having all the cars ever made, all the tracks ever created, allowing you customize or tune everything on a car and having the best looking models will only take you so far if it isnt a good driver.  And that may be where Shift gets them both.  But we will see when they finally all release.