I was going to give my opinion, then I read Khadath and Darth Mobius' comments and I have to say I agree with them. My opinion is pretty much the same as that, you've wrote this asking for help but it seems to me that you'd be much happier with the other girl. It seems that you have more in common with her and the fact that you're entertaining these thoughts suggests you aren't as in love with your girlfriend as you once were. I voted to stay with your girlfriend in the poll but thinking about it I made the wrong choice. Don't get me wrong, I'm a romantic at heart and I've always hated cheaters, very few things make me as angry as finding out someone has cheated on their partner with someone else. However, in this instance it's not that simple, and I'd say it's much better for everybody if you break up with your girlfriend and ask out this other girl instead, but of course don't break up with her in a way that makes things really bad between you. Try and let her down gently, is what I mean, obviously she'll be upset but in the long run, if you feel like this and you have these doubts then if you stay with her you'll likely end up hurting her even more than you will be leaving her. Normally I would say try and make a go of things but this situation suggests, from what you've said, that you love this other girl even more, and that's something that won't change, believe me.
Sorry about the Wall'O'Text, by the way.