Soul Mastery not in the running? 
SM has a good AOE in Dark Obliteration. Gloom is great for Single target and also has a taunt effect. I use it to pull Rom in ITF and heroes in RSFs.
Darkest Night is useful for its to-hit debuff.
If you want to maximize your AoE damage, then Mu Mastery is the obvious choice with two AoE attacks. I'd strongly suggest Soul Mastery, however, because besides AoE it also has a great ST attack in Gloom, and as an ElM toon you're severely lacking in that department.
I would not recommend Pyre Mastery except for thematic reasons, because although Fireball is stronger than any other single AoE available in the pools, the two AoEs in Mu Mastery beat it together. Fireball also costs you one power pick extra.
Soul Mastery looks tempting, but I'm worried about endurance costs. The final build's going to have something like 6 toggles running constantly, and there's only so much IOs can do.
Well you don't have to run Darkest Night all the time, you know. I use it only in the toughest fights. Except on my /ElA brute since he has unlimited endurance, with him I use it every time I can be bothered to not smash things for the duration of the casting time.
The question's pretty much self-explanatory. I've got an Electric Melee/Shield Defense Brute nearing its forties, and I'm looking at the various options. I'd like to maximize my AoE damage, and I've heard good things about both Pyre and Mu Mastery, but I'd like to hear from the board's collective wisdom before I decide.