Why use just one or the other, or even just one of each?
I have a MacBook, a Win95 Laptop (it did have ME on it when I got it, but needless to say I quickly nuked that and put 95 on it instead as it is a much better OS), a Win XP Pro Desktop (hooked to to my 32" LCD TV), and a 24" iMac.
Thats what I call basic, but I'll probably be adding a Linux box or two to it before the year is out, and maybe get a newer windows laptop to run alongside everything else, and then by the time I've got another Windows Desktop (probably with Vista), and a MacPro, both of which will be custom specs as normal, then I'll be heading towards perfection.