I came from SME and don't regret the change!
I wouldn't say that NS is a good "upgrade" of SME. The only thing they have in common anymore is the e-smith templating.
SME is a good an stable server, but here the community is so great.
You won't have to wait long for help or any replay else.
NS is easier to handle IMO, more modern, more active in development and more innovative.
So it's a good idea to change from SME to NS for many resons.
But maybe I'm a liitle bit biased to.