Wow - this is so incorrect it's scary.
Rex Grossman has some of the worst foot work I've seen since Bledsoe.
He reads downfield first before checking down, and hates to checkdown, whereas Romo is tentative about throwing downfield and checks down to mid-range constantly.
Grossman's mental clock is crap - if he doesn't have 6-7 seconds, he is going to screw up. Romo's is off sometimes, but his release is infinitely better.
Grossman is more accurate downfield, whereas his short game is garbage. Romo is more accurate short to medium, whereas his deep ball drops off in accuracy.
These QBs are far from identical.