Ask yourself whether you seek something serious or something casual. Do you want a hookup situation, or a steady girlfriend, or a long-term partner? Think about where you are in life, and what your goals are beyond these two girls. It can be easy to let your judgment be clouded by passion. It may help to take a step back from the girls in order to understand what you're really after. Maybe you need to focus on your career right now. Maybe you need a certain stability or casualness that only one of these girls can provide. What do you need? Think about what they have in common, and try to understand how they're different. Consider what each of these girls offers, and envision how each of them makes you feel. If you're going to choose, you need to fully understand your choices.  Consider which girl you have better conversations with. Consider who is the most fun, and who inspires you to try new things. Perhaps you enjoy being with one girl more than the other. Think about which girl you can trust. Maybe one girl is a lot of fun, but you can't see yourself building a relationship with her. You may need to consider whether you want something more stable or something more exciting. Determine whether you communicate better with one girl or the other. Communication is incredibly important in any relationship, and you may want to choose the girl with whom you can speak most truthfully. Perhaps one girl makes you feel strong, capable, and worthy, while the other girl belittles you and makes you feel like a stranger to yourself. Maybe one lady makes you feel happy and carefree, while the other only stresses you out. Look for the changes in your own personality when you spend time with each of these girls. Consider which aspects of yourself you want to feed. If you can't bring yourself to give a definitive "Yes!" to either girl, then perhaps you shouldn't be getting serious with either of them. You don't need to force a serious relationship, of course, but it's important to understand why you're having so much trouble choosing. Maybe neither girl is looking for an exclusive relationship, or maybe one of the girls isn't even interested in you! This doesn't mean that you should "play" the girls at the same time; only that the situation might be simpler or stranger than you think. Try talking to the girls (individually) to better understand what they're looking for. Don't be afraid to think outside the box. There are many ways to make relationships work. The important thing is that everyone's on the same page, and that you aren't hurting anyone's feelings.

Summary:
Consider what you're looking for. Analyze your relationship with each girl. Notice how each girl makes you feel about yourself. Be honest with yourself. Look for alternative options.