Actually I think there's a chance they do pick Snowden, but I don't care because it's really of very little consequence. I'm sure Time cares much more about selling magazines than maintaining the political status quo.
The audience of the (dying) magazine industry doesn't come close to the audience of snowden's message. your comment kind of proves that snowden doesn't have a chance, because choosing him won't increase sales.
I think there's actually a very good chance. These are journalists doing the picking after all although their selection will likely have pretty much zero to do with the results of an Internet poll. (The chances of it being Miley Cyrus on the other hand are about as close to zero as you can get.)
Depends on what you mean by "impact", as Stalin and Mao were both responsible for more famine, torture, and death than even Hitler. On the other hand Hitler's "impact" was probably more relevant to Time's audience (in the USA and Great Britain).
Don't forget Hitler!