I would be weary of idealization, you're in this actual relationship and you might still be fantasizing about the could-bes and uncharted territory with the other one. Relationships aren't about practical senses though, so if you really can't justify being with your current significant other after measuring them up, I would move on. It's not fair if you are mentally occupied with someone else, and this girl deserves to be with someone that is certain in their commitment.

Just please deeply consider this, you will be hurting your current partner more, and she has invested much more into you than the other. That investment is a very important tangible thing when compared against this hypothetical relationship. As for the other girl, all you would need to do is frankly say that you perceived her situation as you explained, and are now committed to someone else.

However - if you mean by "dating" that you aren't in a relationship and are still exploring the potential of being in one, it's still fair game as long as you are honest about what this is.

I just would recommend that you use your best judgement. If you will be more content with her and believe you'd giving the current one an opportunity to find a better partner for herself, I would go for it. Just be aware that there will be many instances in your life where you will be comparing your partner to others, and your certainty will often come out of inexplicable connections and love - not some qualities that at a shallow glance, seem obvious.

I say this, and would still hope that you use my advice with much salt, considering your experience is why you're on the thread. I also used to consider social stigmas, and the unspoken codes of words to affect how I view things, and now I just listen to what I think and try to take most things at face-value. However, perception is still extremely important, so be aware of how others may receive you.

Like this apple of your eye, I will usually say exactly what I mean and be honest, even if it means vulnerability - but not everyone knows how to digest that.