Let me repeat... There ARE no 'pure' sims on a console. Every console game is on a sliding scale from terrible to barely passable.
While perhaps S2U won't be as accurate in some regards than GT5, in other regards it will be better. I definitely remember that Shift didn't have the vastly exaggerated drafting that GT5 has. I have seen a LOT about how tire temps and deformation and suspension geometry is factored in for S2U (things that are ignored in GT5), and that tracks have 'marbles' off line. For every advantage that GT5 has over S2U, there will in all likelihood be something that S2U has over GT5.
And the bottom line is, NEITHER of them (nor FM3) is as accurate as something like iRacing. OTOH, you aren't paying through the nose every month to play them! And they have a FAR better selection of cars to drive around in, and, especially in S2U and FM3's case, a better track selection (less American, anyway!).
Tell you what... let's lay off the Carnac the Magnificent pronouncements until the game is actually OUT, eh? And show a little skepticism about your OWN game's physics, too...