I would stick with the new girl.
I hate to illustrate it in this way, but here goes (and I think the basic trust issues are the same): say you're an employer and you have a great employee of fifteen years; you trust her with anything. One day, she tells you she's leaving. You're sad and disappointed and you feel a little betrayed, but you can understand why she might want to go, and you don't put up a fight and let her leave. You even once asked her whether she'd ever come back, and she said no. So you find a new employee who is just as good - maybe better - and you're learning to trust her in the same way you trusted your old employee. Then the old employee suddenly wants to come back. You should tell the old employee that you're sorry but there's no position available, despite her being a highly competent employee. Inside, you just don't quite trust her anymore, not like you once did, and once broken, that level of trust doesn't come back. But you could easily see yourself that same special trust for your new employee.
I guess what I'm trying to say is that I'd be worried the ex would just up and leave again. She did it once and realized that nobody died of a broken heart. Once bitten, twice shy etc. It'll be easier the second time. (I'd also worry - and I may be way off base here - that the ex sees how happy you're getting and wants to sabotage your relationship, although I doubt this is the case.)
Stick with the great girl you have right now. See how that works out. Being successful in our rather odd lives often means looking to the future rather than the past.
Fifteen years is a long time, sure. But you've moved on now, right?
Or perhaps not... You wouldn't be asking us this if you had...
