splined BB (isis and octalink) are going out of fashion because they end up with more failed bearings. The evolution went something like this:
Square taper has a smaller axle, with a smaller interface to the crank. This meant problems with stiffness for increasingly aggressive MTB applications.
The axle was made bigger (and different interface-splines added) to increase crank-axle-crank stiffness.
-- as BB shell did not increase in size, the bearings themselves were made smaller to accommodate the larger axle.
Smaller bearings basically suck, and fail faster. This was evident with the large number of trashed low-mileage isis/octalink BB's.
the next phase involved moving the entire bearing contraption to the outside of the BB shell, so that axle can be bigger (stiffer, and now hollow=lighter) while keeping large bearing size, for durability. This is the external BB system
On an MTB, go with external Bearing setup, on a touring bike, i would use sugino cranks, and a phil wood square taper BB, since the stiffness isnt as big of a concern as it would be in freeride/downhill/all MTN MTB. This would be pretty, standard tech (easily replaceable), and very durable.