Is one better than the other?
Also, I might not have time to grind arenas for the best gear, so which one is better w/ avg. gear?
ALSO, which race (Horde)?

Frost mages are better than warlocks in PvP generally, and much easier to learn. Frost mages are better in average gear. The only somewhat balanced PvP is the tip top of the arena, and that is not where people have average gear or skill. As for race, goblins get a little extra haste (1% cast speed) which is nice, but not mind-blowing. Orcs are good, because they get extra spell damage and stun reduction (they also make very good warlocks because of bonus pet damage). Trolls are alright. They get a nice casting speed buff (kinda like a mini-lust) every three minutes. Forsaken get an extra break from several affects like charm and fear. Blood elves get a silence that restores 6% mana. So out of those, I'd say orc or Troll for PvP purposes, but if you factor in aesthetics, I would personally not choose either.
Also, why horde? come on over to the alliance. we have cookies.

frost mage has more armour options & weapon options but lock at level 70 with multiple minions kicks ass.i'd go with frost mage