Well yeah man. Warlocks and shadow priests have different strengths. Warlocks can nuke you down, or they can spec into survival and win the war of attrition. Of all the classes, warlock is one that can and should reliably beat a shadow priest 1v1.
EDIT: also I’m 99% sure Mind Blast has a 1.5x crit modifier, as opposed to the 2x of every other class. That might be a priest thing all around (eg. Smite can only crit for 1.5x damage, not 2x, etc etc.)
But there truly aren’t many other hard counters to priest, and assuming equal gear, a shadow priest can happily win any skirmish.
I would go with priest for a few reasons: I enjoy having utility and being a hybrid. You may be asked to heal at some point, so if that sounds terrible, just stick with warlock. Personally I love having the option to respec into discipline or just pop out of shadowform and throw some heals around.
Power Word: Shield is particularly nice, along with Focused Casting, to prevent spell pushback. PW:S also prevents warriors from generating rage while it’s on you. Big plus!
Shadowpriests also bring to the table: Blackout stun proc, Mana Burn, Mind Control, blanket Silence, a spammable slow that looks like a friggen awesome laser beam, aoe fear/Psychic Scream of course, Dispel Magic which works both offensively and defensively, and an arsenal of healing spells to be used out of shadowform.
Warlocks are strong and great duelists if spec’d SL/SL, I’m actually mad just typing this, as a shadow priest they are the bane of my existence
In a team setting, if protected, they can self sustain and pump out lots of damage. Destro has a neat shield effect that I forget the details of. Neat! Warlocks in general bring spammable Fear, life and mana drain, huge debuff/dot pressure, and all of their pets bring something strong to pvp: Felhunter purge/counterspell, Succubus Seduction, Voidwalker sac for shield. And if you had any aspirations of raiding as a ranged dps, you’ll be far more likely to do that as a warlock and not a shadow priest. 
I don’t know a ton about warlocks so that’s my surface level knowledge for you. Hope it helps. I still recommend priest, the coolest class in the game.