I had SB in my house, had 2 radios and a Duet. I found the SB software a bit tempermental at times with losing connections to services, having to reinstall services, just refusing to sync between radios, etc. Mind you, most of the time it worked fine, it was just spotty sometimes.
I have recently moved, added Control4 to my house and needed more integration and zones, so I swapped over to Sonos. Even w/o the C4 integration, the Sonos app, services and user programming just seems easier, its more stable, etc.
Does not matter for everyone, but I believe SB uses your wifi, whereas Sonos creates its own "mesh" network to put less strain on your wifi/network.
And as it has been said, SB is discontinued. They have another product now, but its only a radio, and it lost some features too - its called UE Smart Radio or something (UE = Ultimate Ears, a label they recently purchased).
If I had to invest in one platform now from the start, it would be Sonos, hands down.