Go with the 15" and get a nice cinema display to connect it to in the office. That way she has dual screens and hence more screen real estate for about the same price as the 17".
IMO the 15" is more overpriced that the 17" one due to the fact that for, what, $300(?) more you get the 8-hour battery and a 17" screen with 1920x1200 resolution. If she's gonna be working on the road she needs all the screen real estate she can get, high resolution is probably more important in those situations than at the home office. The 15" only has 1440x900 which is a little weak, on high-end PCs you can usually get 1680x1050 and even 1920x1200 on 15" screens.
Yeah, I wouldn't wanna see the carbon footprint for Vista... one reason why it sucks so much power would be all these background processes that keep the hard disk spinning like mad. Search indexer, SuperFetch, Defrag, Defender, virtual memory... plus loads of GPU usage due to the Aero engine, which sets the fan spinning, which consumes even more power... OSX runs cooler and pretty much leave the hard drive alone.Apple's laptops do have he highest usage time per watt ratings that I have seen. In addition I've talked to a few hackbook owners that have reported an increase in usage time over Windows by running OS X on their hardware.