Stop using porn and start banging your wife more often. If your wife was a crazy bitch, or she had cheated on you, or you had reasonable suspicion that she might be now, or even if your marriage was just outright miserable in general...then I'd say fuck it go for it. No kids and wife earns more, who cares. But you're describing a relationship that's decent to good, with a woman who sounds good to great, just that you are in a bit of a rut. You can't throw that away on a piece you barely know.
I agree with Palum too, she could flake sooner than later if something does come out of this, then what? You'd be left holding your dick pending divorce while she bounces back to another guy. Once again, she also sounds like a potential head case and she's definitely going to be flaky to a baseline degree due to age.
Basically there's multiple ways to regret going through with this. Its going to be a lot easier to live with letting her get away than to live with losing everything else when she turns out to be a pain in the ass. Stop trying to convince yourself there's something "wrong" with your marriage because you want this girl, its nonsense. Don't feel guilty or tell your wife either. But if you put this all behind you maybe try to make some changes, in addition to the the less porn/more sex, to get your marriage back in a space where you share some better quality times again.
As far as the girl, you're going to need to dial back the extracurricular communications without being too blunt or drastic. It sounds like you guys have mostly just flirted, nothing like you've talked about how you feel or if you should just act on it. So I'm not sure a "let's just be friends talk" is in order unless she actually tried to escalate. The thing is, if you cut back on the communication/flirting then it will be interesting to see if that causes her to attempt escalation. The best thing about that situation is you said she's seeing someone, so it "should" be easy.
Source: internet opinion guy.