a couple of remarks regarding your question :
(1) fatigue life depends on the type of joined connection (stiffness, notches, material, forces, loading etc.) : there will be a lot of different connection types with different fatigue life in the same construction ! do you know the type of loading ? what forces ? it is almost impossible to calculate this for a bicycle
(2) welding and brazing are both suitable for this application , but one must always remember : "the best weld is no weld"
(3) i believe it is more important to make a choice based on production facts (shrinkage, easy to fabricate, efficiency, skills, speed, ...) rather than fatigue life (this is more important for airplanes than for bikes)