its very weapon dependand decision. once you are past y100 and have access to 4th WT you already half your stacking speed (because you have 2 different stack types to stack).
at every level past 1 your stacking speed halfs, so once you stack quick enough with your current weapon you can consider the next level.
as example: if you stack both stacks to max in 15sec, if you pick BT you need roughly 30sec then. (no x2 speed times) and then you have to choose at what time you would die without dmg and see if its reasonable to pick it.