I don't see a direct link to the user base, but a server becomes an "integrated communication server" and you can also add ejabberd for presence service. Providing mail, phone, presence.
My first impressions after setting up 3 machines very recently with Sail:
With SME/Sail, the general knowledge needed to manage the system is concentrated on SME instead of being spreaded between 2 different OS.
The sound quality and speed are better than Trixbox (TB). The dialplan is leaner and easier to read. It's a good system to use and to see how asterisk works. Configuring phones is a lot more simple with Sail when there are many to manage.
Built-in options are fewer than TB but Sail stands out with options unavailable in TB like clusters (and class of service), and a more direct access to customize the dialplan.
Initial configuration is more difficult with Sail because of the way clusters, trunks and routes are organized. But maybe this is because I was used to FreePBX. Telephony is a "capricious" matter anyway. At first, I made mistakes where I had no problem with TB. After these problems are gotten over with, some getting use to takes place... and to me, it is a well known SME-server, a system I thrust and respect.