It's a very difficult comparison. Cindy Sherman is one of THE photographers of the postmodern era, doing serious and elegant work on representation, particularly of the fairer sex.

Ansel Adams is well, Ansel Adams... In many ways he defines the popular conception of "photography" and of course he was a famed printer and user of big negatives. (I belive most of Sherman's work was on 35mm)

It all comes down to how you define art, I suspect Time put great value on Sherman's intellectual contributions to "art" over Adams work in "photography."