I think you have to question which of them has a greater chance of hitting that ceiling, especially considering opportunity -- who is more likely to be given a large enough role to realize that peak?
Because he fits more easily into a modern NBA team, I think Smart will have a great chance.
I think any guy who wants to be a 20-10 back to the basket big man in the NBA now has to be phenomenally talented, and probably has to be able to play center at least some of the time. That's not Julius Randle. So despite his skillset I think Randle is going to have to try to be more like Lamar Odom (to whom Kobe compared Randle) than Zach Randolph.
Smart has already carved out a role on a decent team, and as his confidence and skill on offense grows I can only imagine his role growing, too.