i love niedermayer and hate keith, but i have to go with keith here.
to my eyes, and i think the world of niedermayer's '03 run, keith was better in 2015. and as kyle mentions above, keith could have all three conn smythes and none of us would have blinked. whereas when niedermayer won the '07 smythe, we all blinked hard.
basically though, it comes down to this for me: niedermayer's best seasons were all in a row, while keith's were more spread out. but if we take out each guy's best three years, your "normal" keith year is going to be a lot higher than your "normal" niedermayer year.
that said, i understand the bouwmeester comparison with pre-'03 niedermayer and at times i've even made it myself, but i don't really like it. bouwmeester's problem is he just isn't a take charge player, he's reactive, and so he uses his speed, his reach, and his very good instincts to play a really good safe game. on the defensive end he covers more than his share of the ice, but i never saw a lot of him making life uncomfortable for the opposition. and as a puck-mover, he makes good solid outlets but he takes what the opposition gives him.
all along, niedermayer was an assertive player, early on often to a fault. ran around a little too much on both ends for lemaire's comfort, gave the puck away too much. but that assertiveness in him is also why as he matured he put it together and became a hall of fame player and bouwmeester became "just" a safe, second pair olympian to pair with a play-pusher (pietrangelo).