I had my mt bike built back up. It has hydraulic disk brakes an xt on the front and an Avid juicy on the back. I have the Avid on the back because the lever allows me to adjust on the fly. I like that a lot. it allows me to change my grip as needed.  I went to the same trails as I have been riding the cross bike on. I still like the larger wheels of the cross bike. I think I want my next mtb to be a 29er. my current mtb is a full suspension DBR XR8. It was nice to have suspension on some of the rocks and roots. I think my speed was faster, but a bit more squirelly than the cross bike. The kicker is the break setup on my mtb. The independent levers are nice as well as the one finger stopping power.
 I think if its dry I will go with the cross bike at Mohican 100. The dbr is 30+ lbs and the cross bike 10 pounds or more lighter. Its a tough choice. The DBR is like riding a couch. It may be a plus on a 100 mile ride.