I credit Mister Atoz with some fine observations where these two similar episodes are concerned, but if I were to make a serious choice I'd have to go with Dagger of the Mind by a very very close margin.
Both had excellent guest stars. One had the spinning light while the other had the spinning bed. Both featured perhaps two of the sexiest, most alluring characters in the entire series. The tie breaker for me is Spock. In one he was an after thought and a dimensionless device for "discovery" whereas in the other, we learned something very important about Vulcans and there was added color as the layers of deceit were uncovered.
Flipping back to "What are Little Girls Made Of".....we got one last cherished look at the old style phasers.
Too bad Eddie Paskey didn't really figure into either of these episodes. Seeing him in a given episode helps me keep it real.