Auto update. Cause moving 40+gigs of songs manually is just not fun. I have smart playlists setup to manage songs that haven't been listened to in a long time. But all of my library fits comfortably on my 80gig Classic. I've had a sync error here or there, but rarely re-occurs.
Manually. I like being able to plug in my iPod without having it sync. Other times I'll just want to upload certain items to the iPod while excluding other items.