SONOS development, bug fixing and feature request implementation cannot possibly be as fast as JRiver's.
Have you spent any time around the Sonos forum? They are pretty responsive.
I'm sorry, but the Gizmo app is no where near Sonos. Just from an eye candy level, the Sonos app is much slicker looking and scales to tablets. Compared to Gizmo's outdated black-and-white icons, it is not even close.
Additionally, you can group and ungroup rooms within the app. Gizmo does not link or unlink Zones.
Sonos has something like 38 integrated streaming services, including all the biggies: Spotify, Amazon, Google, Pandora, Tune In, Rdio, Slacker, etc. Gizmo has none.
Sonos has a much better search function within the app; specifically the way it displays results. It also integrates all your streaming services along with your personal collection into one search.
Sonos allows you to create and edit playlists from within the app. Gizmo does not.
Sonos has integrated alarms and a sleep timer. Gizmo does not.
Sonos allows playback of local files (ie files stored on your phone/tablet). Gizmo does not. In turn though, Gizmo allows you to playback files locally (ie directly on your phone/tablet). Gizmo can also be played off your home network, while Sonos is limited to only your home network.
Now, Sonos is Audio only while Gizmo offers Video and limited Image functionality. The Video support is a huge plus for Gizmo.
The other plus is the view customization of Gizmo. However, it can also almost become a hindrance since it is not associated with Theater View or DLNA server customization. It can be confusing and tedious to set up.
From a functionality stand point, Sonos wins in ease of set up and ability to keep groups in sync. Setting up DLNA services in MC can be a nightmare and sync is difficult to maintain.
MC's biggest advantage is high res Audio playback. Sonos is limited to 16-bit/48kHz while MC can play 24-bit, SACD, and DSD.
Sonos is also limited to their own hardware (hence the restrictions above). Some may see that as an advantage (all inclusive); however, I would much rather pick and choose my individual hardware as well as integrate hardware I already own. MC has the advantage of working with a wide range of hardware (TVs, receivers, DACs, Bluray players, DLNA or Bluetooth speakers, car headsets, other PCs, etc).
Lastly, MC is a full featured Library Manager--ripping, tagging, file management, etc. Sonos is really just for playback; some secondary software is needed for Library organization.
In short, I would say that everything Sonos does, it does better. Where JRiver gains the advantage is that it does so much more (namely HD Audio, Video, and Image and Library Management).