I love Tony Allen, but I feel like sometimes we overrate his defense. Yes he's an excellent on and off ball defender, but he sags off his defender at times way too much to try to go for the steal. His wingspan and his ability to skip around screens is what makes him so invaluable defensively, but I think ultimately, I would say Smart in the future will be a better defender, and possibly DPOY/1st/2nd All Team Defense.
I definitely see Smart having a far superior career than Allen, simply cause Allen wasn't that skilled offensively, and was unable to make certain adjustments necessary in order to score.