Kirston's stats are inflated this year due to teams running at him rather than Tyson Jackson/Glenn Dorsey. If he stays next year, his production would very likely slip. Hell, he could lose his starting spot with Ricky Jean Francois coming back.
RJF & T. Jackson are a lot better. Both will be first day picks. Kirston is a second day pick regardless of whether he comes back next year or not. He's a good college player, but he's not dominant or amazing.