I have seen complaints from people who wish Plex handled music better. If Music streaming is important to you, then that appears to be Subsonic's forte.
Firefly, while dead as a project, is still widely used as most music clients support either DAAP or RSP protocols. There is also a forked rewrite of Firefly and an upgraded multi-threaded version available, see http://en.wikipedia.org/wiki/Firefly_Media_Server for details and links.
As to Plex vs MediaBrowser, the top couple of results from a Google search led me to stick with Plex, but MediaBrowser has features that Plex users have been asking for for years. (In particular parental controls.) A few minutes reading should quickly allow you to choose which is best for your needs.
"Free" as in "free and open source" and "NAS" as in
"network-attached storage". The FreeNAS project was founded in 2005 on the principle
that network storage should be available to the world at no cost and
unencumbered by license restrictions.
FreeNAS is an operating system that can be installed on virtually any hardware platform to share computer data storage over a computer network. FreeNAS is the simplest way to create a centralized and easily-accessible home for your data.