If she uses much data I'd say Virgin.

We visit the US from Europe and need data, last year we had the Virgin Mifi device which worked perfectly all the time.

This year we got the T-Mobile 4g mifi device, I find it pretty poor. The speeds are nothing like advertised most of the time. Occassionaly it's fast, but most of the time it's no faster than Virgin 3g, alot of the time it's a lot slower.