If I'm the Celtics, I'd go with Ball. At 6'6 190, Stevens could get very creative with how he uses Ball's length at a few different positions, especially with the lack of size from Thomas and Bradley.
In other draft news, I'm pleasantly surprised with Texas A&M's Robert Williams to return for a sophomore year despite many pundits pegging him as a lottery pick. He's considered a defensive-whiz, but his extra year+ of college will help Williams develop more of an offensive game and become a high-lottery pick. It's refreshing to see his approach to his pro prospects and I wish him the best at A&M next season.