I wouldn't go for beast at all for a necro. Necros have low AR even with fanat, and the damage increase isn't significant enough, as the RS skill %ED to the skeletons count as skill damage, same as fanat, so they just get added together. To make matters worse, all the necro pets have a 15 frame cooldown between each attack, which is NOT reduced by fanaticism, so the IAS boost isn't helping much either. Skeletons have enough AR that they don't usually miss much, so what little gain they get from added AR is negligible.
If you DO go for a beast anyway, I'd go with the axe because other characters may want to borrow it. Probably with the ATMA eth bug and an 1.07 base too, but I'm pretty cheesy. If you don't go for an ethereal base, I'd recommend a Caduceus, because those last longer before they need repairs. Beware the staffmods, though, they can make for very costly repairs...