Brigands benefit more from the mage group. duh.

(no, seriously, they do).
Anyways, I've never gotten a necro past 20, but through research it would seem that there's zero extra benefit to one over the other. The difference (if any) would be so unnoticable that it wouldn't matter. In our guild, the necros and the conj stay on par with each other wether in the mage group or not. (Obviously they do more DPS, but stay pretty equal with each other.)
(edited because I can't type.)