Linux. S3 sleep doesn't work in Windows, for reasons I haven't bothered to figure out. I only use Windows to play games, so I never had reason to care that it didn't go to sleep.
S3 is a firmware-managed power state. The OS (modulo bugs: for example not being able to reliably enter a sleep state) doesn't have anything to do with its power consumption.
>My machine pulls 1W in S3 sleep . . . it wakes reliably and quickly
On Windows or Linux? (My interest is mainly what it is like on Linux.)