The difficulty in choosing the reward will often come when you're faced with multiple choices that fit your character. Some items are obviously not going to work because you can't equip them or their stats do not in anyway assist you for more wow gold. For instance, a Rogue can't equip Mail or Plate so those rewards are useless. A Rogue also can't really get much benefit out of spell damage and intellect so Cloth and Druid leather are out of the question. So obviously for the Rogue any kind of quest reward that grants Agility, Stamina, Attack Power, Critical Strike Chance, and any other melee orientated stats will be the best choice to get wow gold.

Rewards for quests can vary. Most quests before you hit the level cap will grant experience as a reward (although not all quests grant experience). Along with experience most quests also grant a variety of rewards. Most of the time these rewards are in the form of items, mostly armor and weapons with wow gold. Then, most quests also grant reputation if they're given to you by an NPC of a notable faction. Rewards can also be in the form of reputation only, honor, or unlocking instances.

Then there are quests that give multiple weapon rewards. For casters this is generally easy since the staff is always going to win out (unless it's a Druid only staff) or caster one-hands. For classes like Warriors and Rogues then the question becomes what weapon to pick? Here you'll have to analyze which weapon would give you the biggest upgrade with your current setup. Consider how much weapon skill you currently have, if you'd have to skill up the new weapon, and how much of a benefit does it give you over your current stats of wow gold. If it's not much of an improvement and would require skilling up then consider choosing a reward to sell off.