The Mud Terrains have much larger lugs designed specifically for mud.
The Xterrains are designed as an all around all terrain tire with very good street manners.
So,
If you thing you'll be spending alot of time in the muck, the mud terrains are for you.
If you'll spend more time on the pavement like most of us and you'll see lots of different terrain, Rocks, dirt, sand, wet roads, snow and the like then the Xterrains would be youre best bet.