You've got good recommendations here.
The Boston will do well unless the trails are muddy or very technical. Tim Frericks won Transvulcania in them.
The NB Summit Unknown is a new model that, in terms of specs, should be the closest to the Boston. Seems like a very nice shoe.
The Nike Kiger 4 might be a better option. Light and agile, the upper fits great and the outsole does great on most trails. A very smooth and stable ride.
Another option to consider is the New Balance Fresh Foam Gobi 2.
I'd say try just using the Boston. If you feel you want something more trail-specific, go with the Kiger. It may be the best-riding trail shoe out there.