It's a great hike no matter the route. You've got a little readin' to do, but start in on the Katahdin page here at SP. There are great route descriptions, plenty of photos. I think I've done every possible trail to get to that peak, and they all have their own kind of awesome. Of ever a mountain deserved more than one ascent, it's this one.
Knife's Edge is fine for most people, and fun!
Do read up on parking situations for day hikers. A lot of people have the same idea, and space is limited. There are great alternates: the Brothers, Owl's Head, O-J-I. Doubletop. Get Stephen Clarks's book, a reference I've had since I was 10.
On the other hand, there is a ton a great stuff in the Whites, too