If I were their advisor - note that I didn't say agent, DM would be advised to declare and Butler would be told that he has more to gain by returning.
In DM's case, he has already mastered most of the skills that he will need in the pros. He has good vision, the power to gain YAC, above average blocking skills, can catch the ball, doesn't fumble, and decent but not elite speed. Basically a complete back who won't improve much more with another year in college. There are only so many carries in a RB's body so that any more in college will rob from what he could produce in the pros.
In Butler's case, he has some elite skills but also some areas that will limit his effectiveness in the pros. His height and strength allow him to dominate over college DB's. It is hard to be a go-to guy in the pros when you rely primarily on winning 50/50 balls. The strength and experience of NFL DB's allows them to defend their share of throw it up for grab balls. A good example for those who follow the Vikings is Treadwell. He dominated with strength in college, but can't get enough separation on routes in the pros to be effective. The same issue limits Lazard's pro career chances. Butler runs better routes and has proven that he can catch balls in stride better than Lazard, but he still does not have the route running skills to gain separation in the NFL. He has the athletic ability to develop them, but he could use another year of reps and development in college to be able to secure an active NFL roster spot.
They could both have other factors that influence their decisions. It would be great to have them back, but they have earned the right to move on if it is in their best interests.