any of the priest specs are viable to 2.1
in 2s, it will be easier as disc
in 3s, it doesn’t matter, easier as disc or shadow because holy isn’t popular so unless you have some teammates ready I wouldn’t really plan on it
both disc and shadow are quite straightforward, as shadow if you play shadowplay your role is to pump out damage, if you’re play warr/sp or a wizzard comp your role is to survive between your goes and deal enough damage when you can while dishing out instant cc. more than the class itself it’s all about micromanaging the arena. i haven’t done a lot of 3s as shadow this season yet so take this with a pinch of salt.
if i were you if you can’t get past 1600 i would look at what you’re doing wrong because disc is really capable of reaching the highest ratings along with hpala and rshaman, unless you really don’t like your role as disc and you wanna try something out. but you won’t get miraculously 300 rating by playing shadow coming from disc.