This is probably one of them weirder places to ask this, but in your opinions, what are both class' PvP viability and functions like?
I'd most likely go affliction/destruction on a lock and shadow with some healer spec on a priest.
In PvP, primarily BGs and some arenas here and there, which one is better and most comfortable to play? For instance, destruction warlock's Improved Soulfire talent seems extremely clunky in PvP. Similarly, shadow priests' orbs seem clunky as well.
Are there alternative play styles to the 2 that could potentially make them more flexible? What are their strengths and weaknesses? It's things like these that I'm looking for

Disc priests are amazing, I don't like Shadow right now. Shadow priests are easier for me (rogue) to kill cuz of their lack many stuns and CCs that warlocks have.