As usual, I agree with everything @eyemgh has said.
I’d recommend accepted students day at both schools if you can swing it (I think WPI has at least 4 sessions). They are so different and attract different crowds, but both are excellent schools.
As for the cost difference, I believe NU only guarantees housing for 2 years. Off-campus housing in Boston is expensive and she might end up further from campus than she wants to be. WPI guarantees housing for the first year, but it is usually available for upperclassmen. Housing in Worcester near WPI is plentiful and many houses are handed down year to year.