Somebody should be honest with you here and I guess it will be me. I'm not trying to put you down or hurt your feelings or question you at all. I was 16 once and I know you're feeling like this is a huge decision that will affect the rest of your life and all that bullshit.
But in all honesty, this relationship, regardless of who you choose, probably isn't going to work out for any real length of time. It will probably last a couple months then one of you will realize that you're young and should seize the opportunity to have sex with many people before it's too late. Look, I wish you the best in this and maybe you'll be one of those people who ends up marrying his high school sweetheart. but, probably not. You're young, have fun.
You should stop thinking about it so much. If that one girl is already full of ups and downs then clearly things are going to go downhill fast. Do you really want to spend a year of your life dealing with some crazy bitch? No, you don't. Pick the other one. Still talk to the one you don't pick every now and then so that if shit goes sour you have her to fall back on. If you're thinking about picking the Allie girl because she said she was going crazy without you, then you need to reconsider. It sucks but sometimes in life you have to hurt someones feelings in order to make yourself happy. and, if the girl is already that weird when it comes to your "relationship" then she is only going to get crazier.
Or, pick the one that will fuck you the most. if they will both fuck you the same amount, pick the one that is the hottest. if they are both equally hot, pick the one that is the coolest.