I don't know what happened but I am willing to respond. You should choose what makes you happy (similar interests, ability to be honest, similar physical and emotional needs, expectations for your future). Because something is young , new and exciting doesn't make it better. You have to think about a relationship that is going to fulfill your qualities. If you have to get paper and pencil than so be it. Ultimately someone is going to get hurt so you evaluate your options and only you can do that. Search for where your heart is. Think back on your time together with both. If you can't decide you can't decide either to wait or let you date both of them.
Isolate qualities that are important to you and evaluate them on that scale. Also try to figure own who you are excited to see but don't base this on sexual chemistry alone. That wears off. If truly in your heart the new girl will still be exciting in 10 years than that's great. If it's 10 month that is not your choice. If the girlfriend offers , love, companionship and a healthy long term relationship than that is promising. You probably have an instinct but scared someone will get hurt or you will make the wrong choice
I don't think dating both is okay unless they are open to it which is not healthy or likely.
As far as making the right choice, you take a chance either way. So you have to make that decision but go with your heart. You always take a chance of hurting both. You can't avoid that even by being careful. I would not go into details with either girl. This is your problem so let the down gently and make sure they know it's not anything they did. In the end, you can't make a decision unless you take this chance. You can't go wrong if you go with your heart and somewhat your head.