I've spent very little time with the S3 but I think they are about equal. The customizations to the look and feel of the Sense UIalong with their widgets might be of interest to her. The Facebook integration is nice as well. I like the camera UI and features better on the EVO.
Since you already have an S3 I think it would be better for her because TW has some features that can allow two S3s to interact. Other factors like button placement and how the phone feels when being held (due to its shape), may be more to her liking. I believe there are more accessories for the S3 also.
Looks wise, the EVO in white would be my choice. The display, camera, beats audio, widgets, looks, and SD slot are what makes me love my EVO.
My non-biased recommendation would be the S3 only because it is a little easier to use with one hand and some of the extra software features she might like. If not a white EVO is just as good.
Sent from my EVO using Android Central Forums