Hi
I went with Canon as that was (at the time, ~ 5 years ago) what I could afford / justify. Nikon had just come out with the D100 which wasnot cheap, and I picked up a 2nd hand Canon D30. Knowing a few people with Canon stuff meant I had the opportunity to borrow lenses, etc.
These days I would say that the prices are all similar, and the performance is all within the same ball park (with the bod holding the camera being very important). Go with what feels right to you would be what I would normally say.
Canon and Nikon both have a good selection of lenses. Some of their competitors have quite limited selections. Added to which the main aftermarket supplier (Sigma) has a range geared towards Canon and Nikon, with less support for the others. Not a problem if what you want is available though.
If she is experienced with a Nikon camera then she will probably feel most comfortable with a modern Nikon digital body. If she has a few old Nikon lenses then they will probably work on the digital bodies (exception is the D40 / D40x / D60, but the D50, D70, D80 and upwards will all be fine).
All the best
Keith

