I say what you have to do is decide who you think is more compatible with you based on your likes, dislikes and how well you can get along. That is, providing you're interested in both of them. If not, it IS flattering when someone likes you even when you like someone else but the worst you can do is string someone along. That causes feelings to be hurt and in the end you could lose the girl you like as well if she thinks you may be insensitive for stringing the other girl along in the first place and 'keeping your options open'.
In a case like this you really have to play it safe and if you don't like this other girl that likes you, tell her so in a nice manner. Part of this is taking the chance that the girl YOU like may not like you in return. But if that happens, someone else WILL come along.