I would recommend Soulscourge over that magian staff.
Blood boon is hugely underrated in my opinion. When combined with the Caller's +2 set it will produce far better results than an additional 7 MAB's damage. Not only does it conserve MP, it also adds a multiplier to damage based on how much MP is conserved. This can be as high as double damage, which will increase your overall damage more than a little extra MAB: especially considering Blood Boon works on physical and enhancing blood pacts too.
When I solo Brulo, Grand Fall does 3014 (I don't have an optimal MAB set by any means.) When Blood Boon activates, it does 4521 or 6028:
As far as I know, blood boon works in much the same way as Conserve MP. The more blood boon gear you have the more it will kick in. When it kicks in, the +2 Caller's set increases damage or potency of your blood pacts relative to how much MP is 'conserved' through blood boon. Basically, the more blood boon you can stack, the more often it will kick in and the more chance you have of blood pacts dealing double damage.
I would pick Soulscourge over the Pet MAB staff any day for that very reason. Especially considering it has +5 MAB and +7 Summoning on
as well as blood boon. You might deal slightly more damage with the MAB staff but in the long run you're going to deal more increasing your blood boon activation rate. Much more.
Edit: Which has me wondering if I should be using Diabolos' Rope instead of Caller's Sash... This requires some testing!