I was leaning towards new girl, but I'm guessing she isn't as attractive as your ex.
I vote for coming clean to both girls, see what they think and the pain you've been causing by hiding this. Then live with the horrible consequences if your ex takes you back. But you should never have tried to go back at all, and will learn that very soon.
Both are equally attractive. My ex doesn't want to know. I'm not sure about telling the new girl.