While I have never played either caster dps class past level 30, I have a very clear idea of the differences between mages and warlocks based on my experiences with them.
I may be a bit biased by the races of the locks and mages I am familiar with. But I'll go with it, because it's funny.
Mages: I'm a pretty blood elf! Like a flower!
Warlocks: I think my jaw fell off. I could use it to bludgeon you to death.
Mages can make snowcones, heat up a mug of cocoa, and dazzle small children with pretty light shows. They also turn people into harmless sheep, and have a fun little feather-fall spell.
With traditional elements of fire, ice, and "innocent" arcane energy, their spells don't, in their base nature of power, really scream "evil."
dazzle small children into oblivion.
No offense, locks, but I'm not going to ask you to babysit my mini-tree anytime soon.
Mages have pets that look like they were fermented out of clouds and bubbles, like a misshapen balloon animal. They also have shiny, pretty bracelets.
Warlocks, meanwhile.... well, that imp may LOOK cute and chibi from its tinyness, but have you ever really looked at one up close? Really?
Click on the image to zoom in.
Thus, I believe that all mage gear should be adorned with rainbows and flowers, and warlock gear covered in skulls and the blood of their victims.
...that is all.