The first step to deciding between a mage or warlock is to visit the game's website to watch clips and read up on the dynamics of that game's class system. Visit forums or ask in-game friends for opinions. Check out game-play guides available on websites to determine whether one class's rotation or group role fits with what you want. Try each to see which is more fun.
Mages and warlocks are two of the most popular classes or specializations available in fantasy games, especially massively multiplayer online role-playing games. Individuals who have played similar games have probably developed preferences for certain play styles, so watching game-play clips of others and listening to opinions on streaming websites should give viewers an idea of what both individual and group game play is like for mages and warlocks. Visit some of the many websites dedicated to specific games, especially those that offer guides crafted by reputable players. Such guides usually go into great detail about the mechanics that cause the mage and warlock classes to operate as they do. Read about the in-game history of each class to determine if the flavor of mages or warlocks is more interesting. If playing with a group of friends, check what might be most useful to the group. Ultimately, trying both classes and enjoying one experience over the other is likely the most useful indicator of which one is the best for a player.