Dude...you'll loose one when you go for the other. Unless she's psycho and wants to dick up your relationship...she'll probably either quit talking to you or whatever you 2 have now, it won't be the same.
You could always be a bastard and either go for both or keep your options open and if one doesn't work, u have the other.
I had the same deal about a year ago but I'm still with the girl I chose. The other one got pissed/jealous and we rarely talk now. So, there ya have it.