Uhm...what? There are more than two theories.
Here's one I've seen a few articles on within the past year or so. According to test data (SAT, PISA, and I believe others) on teens, girls who are at the top in science tend to also be at the top in other areas too [1]. Boys who are at the top in science tend to be more ordinary in other areas.
People in general tend to to enjoy more and go into careers in areas they are good at.
Result for boys: boys who are at the top in science are highly likely to want to go into science. It's what they are good at and enjoy.
Result for girls. For the girls at the top in science, though, it is just one of the things they are good at and enjoy, and often they are even better at those other things than they are at science. So top girls in science are more likely than top boys to pick some other area to go into.
Roughly, the top girls tend to be more rounded than the top boys. I've not seen anything on why that might be so.
[1] Anecdotally, that fits me. When I first took the SAT in high school, I got 790 on math, and a terrible 540 on verbal. I took it again to see if I could do better on verbal, and got that up to 640, which is still nothing to write home about but is at least good enough to suggest I might actually be able to write. Math dropped to 750 the second time, which was embarrassing.