I vote for XT as the biggest bang for the buck. XT hubs will probably outlast XTR in a purely touring use but both will work well. In Shimano's MTB lineup, XT and XTR get boron coated bearing races in the hub. Boron is the second hardest substance on earth. A good well built set of 36 spoke wheels built with XT hubs and strong rims (ie. Mavic X717 or Velocity Aerohead or Synergy) will be almost indestructible when properly maintained. XTR, Phil Wood etc. may be incrementally better but all cost considerably more. You have the added benefit of the hubs using parts and technology that are so common that most any LBS can work on the bike if need be in some remote location. You can buy these hubs for less than half the price of an XTR with all of the same basic technology and only slightly heavier.