Roethlisberger and Manning are different types of players. Roethlisberger is a play-maker, Manning hadn't been until this year (and even then, it was overhyped). In his 8 seasons, Roethlisberger has had 6 years with a rating over 90, and 2 years with ratings over 100 (and two others with ratings near 100). In his 8 seasons, Manning has had 2 years of ratings in the low 90's, and the other 6 seasons had his rating in the 70's and 80's. The only major stat category that Manning has ever led is "Interceptions thrown", which he's led twice.
This past season was the first season in which Manning could be called "better" than Roethlisberger. Roethlisberger has been superior to Manning in 7 of their 8 seasons, yet you find it difficult to justify and "true discernible difference" between the two.
Must be my "Steeler bias" getting the best of me.