There's absolutely no way to compare. Coach Bryant accomplished incredible feats during his career.
Many younger members may not realize it but he won Kentucky's only SEC Championship and is still the winningest head coach in Kentucky history.
Coach Saban has done incredible things as well and there is no better Coach in college football.
The one thing that may slightly differentiate the two in my opinion is that Coach Saban wins to a great degree because he is an incredible recruiter and has better players than most any team he faces. Coach Bryant in many many cases took a team without a lot of talent and won championship games against teams that were much bigger and faster. He was able to accomplish this because he was such a damn good coach and the most incredible motivator in college football history.
I'll just say this...in their hey day..there was no better Coach than Coach Bryant and there is no better Coach that Coach Saban.