Hi there. I hope I can help.
I don't know much about the guy, but I suspect you're correct in being a little dubious in his intentions. I don't know him well, and I defer to your judgment in the situation if you feel my assessment is wrong, but I have to remain a little skeptical in that I fear an ulterior motive in him wanting to get back together with you. If you regret having sex with him then, I don't think a repeat of that will ease that, you know?
On the other hand, it sounds to me like you have a more genuine interest in the woman. My only thing I'm a little concerned of is the fact she is a parent. While that's not a
fault or anything at all like that (and indeed, she has my undying respect for being a parent to her first child in regards to the circumstances...truly, she does), but speaking solely for you and you mentioning you are a teen, being a potential stepparent is a LOT of responsibility. That is really my only concern there, and I trust your judgment in that matter. It is only my intention to mention that responsibility you may be taking on for yourself.
Ultimately, I would tell you unless you can truly, honestly say that you wish to be with one over the other, it is best to remain single. You only do yourself a disservice by giving your heart to someone when your heart isn't fully decided.

That, ultimately, is my most important piece of advice.
I wish you all of the best, and I hope I was of some help.
My absolute best,
Harley