Thurman was the better player. But there is NO denying that he had a MUCH better OL, QB, OC, TE's, even WR's, i.e. offense in general, than Willis has had.
And last year in terms of yards, Willis had the 5th best output of any RB since the 16-game schedule went into effect, and the 9th best output in Bills' history, since OJ outrushed him (in 14 games) 3 times.