I disagree that 75% of students have a laptop... I just graduated and in class, there was 1 or 2 student with a laptop... I know a few who had some at home, but its certainly not 75%... I'm quite sure in other programs there are many more users...
As for a laptop, well I can't stand those... they are great for mobility, but that's about it for me... I couldn't have imagined myself doing university on a laptop... So there is also a matter of personnal preference, if it were me, I'd definitively go for a desktop.
As for Vista, well... I'm quite sure its not as bad as people say, at least that's what I read and with SP1 coming, it will fix a few things, won't quite make it perfect, but it works at least ok on a new desktop...