hands down linux.
if you're not affraid of a challenge.
i'm running linux on a p100. it runs www server, ftp server, internet email and a smb (file server), plus other goodies...
with an uptime of about 8 months... i leave home and forget about the thing come back and its still going strong. only time i shut this thing off is once when power went out and my ups only had enough backup juice for about 15minutes.
never had a problem with it.
3:48am up 281 days, 3:23, 1 user, load average: 0.00, 0.00, 0.00
going for a year!!