I have both a 1k+ boomy and Elemental shaman. I am starting to really like my shaman more. I crush my boomy on trash dps now unless there is a seriously big pull that lives a bit then boomy can do well.
My shaman is an engineer, so he also brings battle rez. The only thing the boomy brings utility wise are treats and solar beam. My earth elemental can sub in better than trents for a pull that really needs it or I can use the defensive from it or dps as well on a boss.
I definintely do more damage on mythic bosses as elemental unless it has a shitload of crazy movement such as knight captain Valyri in Tol'dagor or Yazma in AD with volcanic can suck for elemental too... but also sucks for balance cause balance needs to hard cast a lot... only thing I like about balance is it can build AP while moving and all the elemental builders are pretty much hardcasted.
I feel a bit tankier on my elemental now too... between a better defensive on a shorter cooldown, the faux cheat death on a short cooldown, GW on top of it if I know a big hit is coming + earth elemental--- I am usually the last to die in a group (pugging).
So elemental doesn't have solar beam but it does have the shortest cooldown on an interrupt of any class and its ranged. In some cases its better than solar beam, such as the tide sage council in shrine or certain fortified trash.
Elemental definitely shines on having snap burst on trash as well, an area balance suffers. A group has to know how to pull for balance for balance to excel.
So all in all, balance edges out in off heals, mobility, and treants on a necrotic weak. Also the solar beam is definitely amazing when used correctly.
I'd say elemental does better in a pug setting, the burst damage is nasty, single target is very strong- no sacrifices in spec for M+ (AoE vs ST), provides the best knockback, has an AoE stun, the best interrupt, is pretty tanky, can ankh once or twice too depending on traits, and provides AoE snare+knockdowns. Be an engineer so you also bring battlerez.
Mage/boomy have less to micromanage so might be easier but I think elemental is extremely strong in M+ and is only going to get better with scaling/new traits.
I still like my boomy but notice im doing M+ a lot more on my shaman than anyone else now. (1.3k .io on both, don't have the consistent team or motivation to push higher)
its starting to creep into raider.io too.. check out
https://raider.io/characters/eu/tarren-mill/Djfroosh
#23 DPS in the world is an elemental shaman.
finally, using balance as my comparison kinda hits like a wet noodle outside of incantation but its a super amazing/strong cooldown. Elemental I feel has a lot more mini CD's to have strong numbers even if the elemental is down.
but shhhhhh don't tell anyone.. I like how ele is a sleeper in M+
Right now I am just praying we don't get nerfed. Its rare elemental gets to be this good in PVE/M+ - also, I have no trouble getting accepted into pugs. for +10,11,12,13,14+ etc keys