It depends! Physically, clitoral stimulation is easily better,but it's easy for her to distract herself out of a good orgasm when she worries about looking pretty or if she's taking too long to cum. If she's comfortable around you or if you use toys, that's how you can really rock her world.

However, sometimes with vaginal stimulation, she enjoys it more overall because she worries less about not being pleasing to her partner. She might still get an orgasm if her partner hits all the right spots by brute force or by accident. Plus she gets a lot of mental pleasure from being penetrated.