RHEL/Centos, running mongrel cluster and haproxy. Rails uses Oracle, which lives on a dedicated server, but the rails server also contains a postgresql and mysql db (and a shitload of other apps). Average uptime: 400+ days, last downtime was to add extra ram. 1 unexpected reboot as a result of my own stupidity.
I also have another sever with the same combo as above (except the rails app uses mysql instead of oracle, but postgresql is also on it, might switch to postgresql in the future, but mysql feels safer because it's being used so much). Uptime last time I checked was 106 days or so, so far no unexpected/unplanned downtime.
My advice:Use what you're comfortable with (but learn linux if you're only comfortable with windows).
Piet.