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 don’t 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, you’ll find its easier to get a spot in content as a Boomkin than a Shadow Priest.