Brooks. But, for a mountain bike, I'd suggest a narrower model than a B-17. Consider the B-17N or, even better, the Conquest. The Conquest has springs in the rear and makes a poor man's rear suspension.
The narrower Brooks models are good on a mountain bike because it's a bit easier to slide behind the seat when you need to shift your center of gravity. If all you do is ride on streets, then a B-17 by all means is the best choice.