In a keeper league who has more value for this year? Who will be the better pro?

This year? Rasmus, I think he's the first to get called up by several weeks. Best pro? Don't know enough about Rasmus and Bruce defensively to answer that.

This year? Maybin. Doesn't have much in front of him, Marlins are committed to being bad, and even if he struggles to get on for awhile, he will run a bunch.
Unfortunately for Rasmus and Bruce, their respective managers don't like younger players, so they may not get ample opportunity to play this year.
For the future, I think Bruce will be the best of the bunch. I like Maybin slightly better than Rasmus, just because I think Maybin has higher 'upside'. If Maybin reaches/exceeds potential he will be the best. I don't think he will, but he has a chance.