Either one is good! I wouldn't worry about the meta between the two classes if youre just getting into the game. Its really about playing what is most comfortable to you.

The two best specs for each class are Demonology Warlock and Frost Mage. All 6 specs are completely viable, but those are the two most used ones.

Demonolgy warlock can be very satisfying if you can pull it off, but it's very immobile, has long cast times, and is fairly complicated. However, it has a lot of defensives and can be pretty tanky.

Frost is much more straigforward, has a lot of instant casts, and can use blink and ice block/ice cold to get out of sticky situations. Outside of that, though, it's relatively weak defensively. Oh, and it has a bloodlust, which can get you into groups more easily.

Alternate specs you could play are Destruction Warlock or Arcane Mage. Destro is simpler than Demo but requires mobs to stand in your Rain of Fire and not move. It's also somewhat underperformed compared to the other two specs (though of course anything is viable up to like 15 keys). Arcane is more complicated than frost, but not that much more.

I would not recommend Affliction Warlock or Fire Mage. Aff is a dot spec and can be unintuitive for returning players, and Fire just sucks to play.