I'll have to keep that in mind
Best analogy i could think of. If you look at the two units on a charge, yes you get many more attacks from the berzerkers but at AP- unless you spend the points for chain axes.
Zerkers - 4 WS5, S5, AP- Attacks - 19ppm
Bloodleters - 2 WS5, S5, AP3 Attacks - 10ppm
For double the cost you get twice as many attacks (great!) and power armor (but no invuln like the letters)...and i guess another toughness as well but the lack of AP is killer. Though my analysis could be heavily skewed based on the fact that i fight necrons 98% of the time. The gap is probably a lot smaller if you are fighting lightly armored dudes who aren't throwing AP3 attacks themselves. Still though, i think i'd take bloodletters for the DS daemon synergy.