Just a heads up that Mac OS Sierra has some major issues:

• windows redrawing slowly and funny glitches on my screen. (still not fixed)
• time machine backup is broken –  I cannot backup or restore from time machine back up.  (still no fix. Apple have some suggestions about  resetting the PRAM but that didn’t seem to change anything.)
• downloads not working in Safari –  they stop before the download has completed. (now fixed – Dec 2016)

So if you are thinking of upgrading to Mac OS Sierra, I would be holding off until at least the first software update.