The difference in minutes on the court is scant compared to the amount of practice time you get in the pros, the lack of school responsibilities, the absence of limitations on practice and contact with coaches, and the fact you're playing against better competition in practice.
Each situation is different. Lamb had to go because of his stock, but may have developed quicker with another year in college. But if you're a lottery pick, you have to go. Butler had to go. Drummond had to go. Thabeet had to go. I don't know that there's been a coach who has gotten better information and given better advice to his NBA prospects than Jim Calhoun. I have full confidence in Ollie to do the same. He's too well liked, played for too many teams, and has too many contacts to not be able to get quality feedback from scouts/front offices.
Daniels isn't a lottery pick, obviously. He has first round talent, but that doesn't mean he'll be a first round pick. It's a risk/reward. The way I see it, another year of college can only help him. His stock isn't going to get lower, only higher. God forbid he gets injured, he should be on the path to graduating and will have a degree to fall back on. He'll be an all-star in any league not named the NBA whether he comes out now or next year, but another year could get him that guaranteed contract and a couple million dollars.
Whatever he decides, I'll be a fan for live after he helped bring home #4.