The classic optimizer is recommending the Nightslayer helm over the Bloodfang helm. I presume it’s because of the 5 piece set bonus (+10 maximum energy) but is that really worth more than the +7 agility, +6 stamina and the resistances?
I feel good about the rogue calculation in the classic optimizer, so I’d follow it without worrying. Those two items are almost equivalent - like within 0.03% of each other. You could use either and never notice a difference. Nightslayer also has that extra 1% chance to crit.