Depends on what you want to do really. Since they’re both casters, they’re kind of the same gear-wise, until you get to level 70.
Mages can get boring but they are really strong DPS for both PvE and PvP. Also they get to make water/food if you want to save some money that way.
Warlocks are a bit more complicated because there are a few types, depending on what you want to do. They’re really good at PvP because they can chain fear and do DoT’s (damage over time). Locks are also good for soloing because they have pets to tank. You also practically get a free horse at level 40 and a cool looking horse at level 60.
Gnomes are both great for these classes because you get a racial bonus in intellect meaning you would have more mana.
It really depends on wether you want to do burst and rapid damage (mage) or damage overtime (Locks).

well… in my opinion i like mages more but again thats only me… mages have alot of power and warlocks do to but seriously Dotting people requires no skill and if you reallyt want to enjoy the game, choose somethingthat requires skill and practice to get better—

hope this helps