I'm not one to say a player "should" go pro. That's up to the individual player. I instead focus on the marginal benefit of returning, and how the player's motive for returning affects that benefit.
Given each player's position and current scouting reports, Butler stands to improve his stock if he can come back and show he can cut down on the drops. He's probably a third or fourth rounder right now, and receivers go in the first round much more frequently than running backs these days. So there's definitely a potential NFL benefit for Butler to come back.
Montgomery, on the other hand, isn't likely to improve on his weaknesses by coming back. He's a very technically sound player, but probably lacks the athleticism to improve his stock beyond the third or fourth rounds, which is where he's already projected. NFL teams are now rarely spending first rounds picks for any but the truly prodigious talents (Gurley, Barkley, McCaffrey, Fournette, Zeke, etc.). They know they can get guys like Alvin Kamara, Tarik Cohen, Kareem Hunt, Lamar Miller, James Conner, etc. in the middle rounds. That's the kind of guy Montgomery would be in the NFL: valuable, productive, but able to be had without spending a first or second round pick.
If Montgomery wants to come back (whether it's because he wants to graduate, or he just loves the college experience, or just wants to play one more year with his guys, whatever), he should, because he can get what he wants by coming back. But he shouldn't come back under the impression that he can improve his draft stock. Because he's likely peaked in that regard, and he's just wasting a year of mileage on his legs if that's why he comes back.