Congratulations! Those are some wonderful choices!!!
For those majors, I'd definitely pick either MIT or Princeton. Stanford is good too but MIT and Princeton are another level up. I've only been fortunate enough to visit Princeton's campus (DH was giving a talk there). It's amazing!!! I really love the city and the campus too for that matter.
What would probably help her the most is go to the websites of each university and see what the professors specialize in (and if there are any areas that she is particularly interested in). Maybe she could even try and talk to those professors or at least a postdoc/grad student of theirs to get a feel of what it's like. Sometimes the students can be honest (REALLY honest, when I was look at grad schools one student flat out told me not to go to the university I was looking at because it wasn't at a high enough research level for me).
If she does plan on going to grad school that degree is the one that's ultimately going to get her a job. However, a good undergraduate degree can really help to get into a good grad school! Good luck!!!
