OH man, that's tough! I don't understand the advice about "do what will make you happiest" because you don't KNOW what it's going to be like with the new one. If you knew, you'd do something already.
Three questions to ask yourself:
1. What's going on with your current relationship that makes a new one attractive? What's wrong with it- or more precisely, what's wrong with how it feels to you?
2. What do you *imagine* it would be like with this new girl- it sounds like you don't really know her very well, but want to. Why? If it's just her body, better look deeper if you're interested in the long term.
3. What kind of a person are you with the current girl, and what kind with the new girl? Which brings out the best in you? It's all about who you are when you're with her, not about who *she* is, IMO.
OK, so that's more than three questions. You say there isn't any real relationship right now with the second girl, which suggests this is much more about you then about her at this point. Why not just get to know this other girl a little better- be friends for a while if you can? Time together is the only answer in my book...