Becca said it. You're not being fair to the current gf.
That being said, the one in France definitely sounds like a "what if" like you said. You didn't get the chance, so now you fantasize or obsess about it just because it didn't happen.
Ask yourself honestly, though, did you guys really have anything in common? Similar tastes, likes, dislikes, etc., or was it just a mutual attraction and nothing else?
If it's based solely on attraction, you may end up losing a good gf simply based on over-glorification. What's to say if you guys go out you won't end up fighting or bickering after a week, or even hating it and breaking up by then, or a month from then?
It does sound like you have a choice to make, though. Ruin what you have (which you will if you chase after a dream) or go for the long shot. If it's been nagging at you that long, it is something that definitely needs addressed sooner rather than later.
One of two things will happen. Either you will screw up a good thing (it happens; we all do it at one time or another) or you'll have found something that will never get old, always make you happy, and be well worth the risk.
Either way you go, suck up the consequences like a man and chalk it up to a life's lesson. If you don't, and try to go back to the current gf it will be a shell of what it used to be and you will be comfortable until your next "dilemma" which, while not only unfair to her, shows that you really don't want to be with her if you're constantly wondering "what if" about others. Almost sounds like you're happy w/her (at the present time) until what you really want comes along and that's not considering her feelings or the time she's losing by being w/you, much less being fair to yourself.
If neither situation works, then neither of them were meant to be, and just continue in your life a little wiser and a little more self-aware.