The original regulated voltage of the alt doesn't really matter since the regulator gets bypassed anyways, but the 24v alt's are usually a larger alternator since they mostly come on heavier duty vehicles. Physically bigger and higher amps the alternator is, longer it should last in this application since the heat is what's going to kill it. Also cost quite a bit more. I've got about $140 in the alt and pulley.
Lot of threads over on Pirate about these with different alternators, the old Ford truck ones being common but they only put out 70-80amps.