Analyze your relationship with both girls. What do you actually want from each of the girls? Just a physical relation? Marriage? A fun time? Soul mate? This is a very important decision, or you'll be stuck with something you don't want.
What does one girl have that the other one doesn't? If both are good looking, which one do you enjoy being with more? Okay look, we all know the hotter one usually wins, but if you're unhappy with that boring, one-faceted girl, it's not worth it.
Make a table with one column for each girl, and list as many good qualities as you can think of.
Locate any qualities that both girls share, and cross them off the list. You are now left with a list of unique qualities that each girl has. Review these qualities and compare which ones you like best. The girl with the most and best unique qualities is most likely the one you should choose.
Once you make your decision, be direct about it. Girls love it when you're direct. Go up to her and just tell her, "I can't bear to be without you." She'll melt. They go for that stuff, once in a while.