I pushed my spriest to 216il. Got 1600cr in everything but 2s on it this xpack and chill in m10-12s. I will 100%say boomkin is better and will out class you. Just go Boomie.
In my very biased opinion, go Druid.
They can fill in literally any role.
If you get tired of ranged dps, you have 3 more options without having to switch classes and most gear swaps Intel and Agility.
Main SP here. I think the way Shadow plays is top notch. Rotation, choices in talents, etc are as good as I can remember Shadow ever being. Very fun class to play in my opinion.
Having said that, we donât compete well against Boomkin. Boomkin DPS is top tier on all damage profiles (single target, cleave, aoe), their utility in M+ is bonkers (trees, roots, vortex, battle res, etc) and their DPS in raids was almost mandatory on certain fights (convoke broken and starfall on council is a must).
If you like both, youâll find its easier to get a spot in content as a Boomkin than a Shadow Priest.
If youâre still on the fence, Iâd wait a bit at this stage. Boomies on the ptr are paying the hybrid tax again doing support level dps. Shadow priest had their aoe buffed and single target nerfed, so Iâd just hold out to see if anything else changes.
Shouldâve given it a shot during legion/BfA. Dot management was automatic thanks to void bolt and it was one of the fastest specs with the stacking haste buff. But alas, those day are gone 
There is a talent row where you can set Vampire touch with SW:pain at the same time. It is very handy and donât feel that you are managing any dot.