the thing about the ipod and the mini is that it will only sync with one computer. if you plug it into your mac it will copy all the songs off itunes, but if you plug it into your windows pc it will ask if it can format the ipod so that it can be read by a pc, and then copy all your windows songs onto it.
i suggest putting all your songs on your mac and keeping your ipod synced with that. i say the mac over windows because with the mac you can use isync to put your ical calendar and address book contacts onto your ipod.
depending on how much music you have, you could go with the mini if you have under 4 gig, other wise go for a normal ipod.
enjoy