I have to admit I think Frank will be better at defending than DSJ, the reason his height and those freakishly long arms. DSJ may get you more highlight blocks than Frank but his height and those long arms are going allow him to affect more shots than DSJ can.
I was watching Kris Dunn and was impressed with his defense and the speed and athleticism he has but his career might not go anywhere because he still can't shoot. That could happen to Frank as well so its still too early for me to say he's the Knicks PG for the next 10 years, he has shown court vision, passing and knocked down some shots and a lot of promise so I'm hoping.