Size does not equal height or length, it equals bulk. So to recap, Jimmer has better size, a better shot, and better ball handling, all while being able to be had 9 picks later.
That said, I don't dislike Knight, he brings plenty to the table and he's only 18. I think Jimmer is the better value and that his basement is much higher, but Knight has a higher ceiling and is further along than Jimmer at the same age.
Length + Height > Size when it comes to a PG.