你好啊？ or 你好吗？ Which is more common?
Usually people just say 你好 because, well, it's just the common way. I don't think there are any reasons.

你好啊 sounds like talking to babies, or if you are greeting someone who is some distance away and you want yourself to be heard. 你——好——啊——

你好吗 is a direct translation of "How are you?" It is only used by Chinese when one really wants to know what is going on with the other, or it may be used with English speakers to make them feel less culturally alienated.