Boss dmg 20% or ma+70 ?

I'm not too knowledgeable on the attack/casting speeds of the standard BaM but I believe you'll be better off with the faster staff. Reasoning below:

FB and your hyper (6) = 810ms +240ms

Speed(8-4=4) = 720 + 240 = 1.04 cycles in 1s
Speed(5-4=2, if we consider speed cap) = 630 + 210 = 1.19 cycles in 1s
Speed(5-5=0, using a green mpe pot or alternatively BT's card which is actually -2) = 510 + 180 = 1.44 cycles in 1s

Speed(0) is a 38% increase in total output compared to speed(4)
Speed(2) is a 14% increase in total output compared to speed(4)

Bossing and Total damage% multipliers tend to drop off with the more you have.. so it's safe to assume you're better off with the faster staff. Though again, I'm not familiar with the class so I apologise if I've mistaken something.

BaM hits speed cap with just self buffs using a slow staff. With MPE green pots breaking the cap, BaM gets pretty far into negative attack speeds when using an E Staff.