KG is the better defender. The only thing Perk guards better are centers that pound the ball inside. In all other scenarios pretty much KG is better. He can guard effectively a multitude of positions, he can guard the post, the perimeter, etc.
As for Rondo "Second Team Defense" is crap to me. He really only got there because of the amount of steals he gets. This is where he's good at... doing full court presses and half court traps, playing the passing lane, and sneaking up behind big men on the post to strip the ball away from him. He also has good hands on him to reach in.
But sadly, the thing he fails at is the most important aspect of a defender, particularly a perimeter defender, particularly from the PG position, and that's keeping his man in front of him, stopping PG from getting to the paint... stopping penetration. He has been consistently subpar in this.