In 2013, Sullinger played half of the season with a herniated disk that required surgery and averaged 6 and 6 in limited minutes...
In 2014, he's averaging 13 and 8 without a proper training camp and while playing out of position in 27 MPG...
Sullinger has the ability to average a double double (he'd already average it, if he were a more selfish player (he'll actually box out opponents until a teammate secures the rebound)), is already displaying a solid understanding of help and post defense, is just as strong as some Centers (imagine him in the post against 4's, when the Celtics finally play him in his proper position), and he just turned 22 years old.
Randle is an impressive player, but he's an unknown commodity at the NBA level. At the college level, he has defense and serious turnover issues that will keep him off of the court for chunks of time. No one knows how his style and attributes will translate they can only project.
Randle could eventually be better than Sully, but it won't be enough of a gap to justify using the C's 2014 1st round pick. If Ainge can't get what he projects to be a potential elite player at another position, he should package the pick in a trade.