Depends upon what you want from a player. Anderson was primarily a scorer in college, looking for big stats. He's athletic so he can be taught to play more defense--but let's face it, most scorers have concentrated all through their careers on their offense and defense took a back seat. Anderson will have to learn to play defense to get a lot more time in the rotation. I hope he 'gets it.'
Leonard on the other hand, like Bowen and some few NBA players, has always been known as a hard-nosed defender and rebounder. On a team like theSpurs with a need for a defensive stopper, he may fit into Pop's rotation quicker than the average rookie. Learning the Spurs defensive system may be difficult--more so for the scorer type-- and some never 'get it" because they've never put much effort into that phase of the game.
However players who WANT to play defense, have always concerntrated on the defensive side of the game, and pride themselves on it can probably pick the system up quicker.
Also an athlete with Leonard's attributes--long arms, big hands, relentless pressure--can often make up for mental errors he makes in system rotations.
Between the two, I think Leonard fills a bigger hole in the Spurs overall needs than Anderson and may get more time on court.