The logical two would be the two with the least playing time at the Division 1 level, and that would be Brock and Pruitt. Sure it would be nice to get Randle some much needed court time, but he's been in the system now and he will not be as far behind as Brock and Pruitt. Arnold, on the other hand, played two years at Illinois State and participated in practice all season long.
Depending on recruiting, there's a chance that Brock will need to learn how to become more of a guard than forward, which is where he played in high school. Also, I'm sure Weber will experiment with him there and give him a chance to work on his shot. From an athletic standpoint Brock and Randle can compete with anyone in the country, but Brock will need that game time experience.
Pruitt is a big man that has a tremendous upside to his game and will be needed this year if the Illini are going to keep pace in the Big Ten. Pruitt will have to replace minutes from Jack Ingram, Roger Powell and Nick Smith. Physically, Pruitt will be able to hold his own in the Big Ten, but he will also need to go out against other players in a game setting to see what he needs to work on.
I've had the chance to see Arnold in some pickup games and he's going to be a fine addition to next year's team, as well as Randle. Before Randle's injury he was playing some of the best basketball on the team.
Weber will have some time to think about his two candidates. You can't go wrong, but nothing takes the place of game experience.