Lemme tell you a lil about both:
BOTH are the pinnacle of casters in terms of strength.
But historically if you compare them Mages are far ahead in M+ and PvP, while Warlock often is stronger than mage in Raid. That being said… since you need an int-buff Mages will never ever get a problem getting invites.
So if you want to be allways strong Mage is the winner (Both are historically strong)
Mages overall are mobile, have high controll, high damage in ST and AOE. They are extremely tanky if you know what you are doing, but frail if you are a bad player. They also have great utility for the group.
If you wanna play mage, get used to play fire, because thats the spec that is allmost allways the best spec for them since a very long time. The specs also are way too similar in terms of gameplay. (Arcane being different tho)
Warlocks are immobile, have good controll, high damage and are extremely tanky in all scenarios. They have great utilty but usually are more restricted due to it being attached to a demon or a curse.
If you wanna play a warlock, you have multiple specs that play very differently because one is a pet-spec, one is a dot spec and one is a direct-dmg spec, so overall more enjoyable if you like to play multiple specs. That being said from a balance point of view usually one goes extremely ahead, while the others lack.
If you dont do high content I would suggest you playing a warlock. If you wanna aim for the stars you mostlikely will be better of playing mage. If you play semi-high you cant do wrong with either (as in general)
At the end its up to what you like more.
(This feedback is kept pretty fundamental to not go too big)