Depends
It depends on your earning and redeeming circumstances. If your stays are mostly short ones, then HH is a faster program as you get the bonus miles per STAY, not per night. But if you tend to have longer stay, go for MR.
In terms of redeeming points, Hilton has a better value for award of 6 or more nights (GLONP2). MR does much better on award of fewer nights stay though.
Both have merits in different ways. You should join/keep both programs. But if you are to choose only one, look at your situation and preference.