I'm a big fan of cones in general. My crab works a lot like your brute: two cones to start things off, then grenades once they're all clustered around me. The thing is, the crab can drop an AoE immob on the mobs and then back out for more cone action. Can your brute?
I go for Mu on my AoE brutes, because it has two ranged targeted AoEs. When I've got mobs clustered around me, I don't want to have to switch from melee to range and back and forth to get the most out of my AoEs: I want to be able to lay down a lot of AoE hurt in melee range, over and over. Ranged targeted AoE gives me that. Cones wouldn't.
If you consistently fight things that drop after arctic, bile and FS, then you're probably good with Leviathan. I really do enjoy puking all over mobs.
Scrap
Well, indeed, I'd say if your current Leviathan attack chain is mopping the floor really well as you get into the mob, then there isn't a need to shift over to Mu mastery for its AoEs. Mu mastery's AoEs work well when the mob is clumped around you already and less overall damage then all the cones in Leviathan.
+ Points though:
Mu:
-Both AoEs (ball lightning and fences) are easy to use in melee range.
-Fences has a -kb component along with its damage and immob, which may be a blessing or a curse. It can keep enemies clumped and allow you to use burn more effectively, but your KD powers won't provide the mitigation they usually do.
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I like the cones from Leviathon Mastery and have them on a couple of my brutes. I get the best use out of them by jumping straight up while in a mob so the cone fire straight down and acts like a AoE then. It works best if you have hurdle for the extra jump height.
My SS/Fire took Mu and never looked back. I actually respecced into MU from mace when the respec became possible.
Mu's Targetted AoE immobilize is THE power I need on my SS Fire. Ball lightning is a plus.
Ball Lightning>Footstomp>Elec Fences>BURN
Burn does amazing aoe damage once you max out it's damage and recharge. The only problem is, it scatters mobs all over the place. Things stay right where they are with fences, and eveything melts away.
Which is the better patron pool for AoE damage.
Currently I have my SS/Fire brute going the interesting route of having Mako as a patron for Spirit Shark (does good damage in PvP), Bile Spray and Arctic spray.
Normal opening AoE attack chain goes Arctic spray, Bile Spray then when everyone is clustered around, footstomp.
This is enough to wipe out a group of even con minions and with Fiery embrace up +1 lts.
Would another patron be better to go with?