You'd only have to be 85 or so to have been old enough to remember seeing Sewell play. It's feasible. Though there can't be many guys who both saw Sewell play and are posting on Internet message boards.
I'd take Mattingly, because he was far scarier during the few years he was great than Clark ever was. Clark wasn't ever really that much better than Keith Hernandez, and if Keith Hernandez was the best hitter on your team you were screwed.
Of course, you can imagine how overrated Mattingly was by the New York media though. Donnie Baseball. He wasn't even the best player on his own team the year he won his MVP. (Rickey was.)