Ke Jie just happens to be #1 right now. Lee Sedol is more impressive in that he's been a world-class Go player for much longer. Twenty years from now, if Ke Jie is as dominant as Lee Sedol is now, then he'd be a good pick then. But either way it's not a big deal; there's not that much daylight between the two, so picking the more famous and well-known one to go up against is a safe bet.
Personally I want to see a discussion game with the top Go experts (including both Lee Sedol and Ke Jie amongst others) competing against the next version of AlphaGo, in a game with much longer time limits.
I don't think top pros view it that way. He is of a new generation that play thousands of games over the internet; that study the game socially, while Lee is much more a loner. Whether Ke Jie will dominate as much as Lee did will say more about the strength of his competitors than his strength vs Lee.
Personally I want to see a discussion game with the top Go experts (including both Lee Sedol and Ke Jie amongst others) competing against the next version of AlphaGo, in a game with much longer time limits.