Let's see, I think you all are in dire need of a female perspective here (no offense, but you seem to be floundering).
While I'm sure many girls wouldn't appreciate being compared to a box of chocolates, I, for one, would take it as a compliment. Perhaps unpredictability is one of the things you should enjoy about your girlfriend, instead of begrudging the fact that she doesn't always react how you'd like her to. After all, if you knew exactly what she was going to say, it'd be pretty boring.
And I agree, don't argue, discuss. Just because you don't understand each others' point of view right off the bat doesn't mean that with a little work, you can have a working relationship.
And by all means, listen to what she says! Perhaps she can't back up her beliefs with scientific proof as you'd like her to, but maybe she wants you to have convictions that you can't quite explain. Opposites attract. Compliment each other, give a little, take a little.
Communication is vital-and if you're willing to compromise, she should be too.
Good luck