I would lean towards SL lock due to the utility/survivability and scaling very well with improvements in gear, but a tryhard spriest would do gnarlier damage earlier on, and has dispel spam.
I think people in general are more intimidated by shadow priests too, but once you're oom you're a free kill
so uh, if you wanna go ham with lots of buttons and feel so action, roll warlock
if you wanna play defensive support with some burst, and get cried at for not healing all the time, then spriest
oh and you'll be stuck farming demonic runes either way, so factor in farm capability