Plex requires running server software, Infuse (or MrMC) doesn't. This is a big advantage for NAS users.
You can also buy a perpetual license for the current version with a one-time payment:
https://itunes.apple.com/us/app/infuse-pro-5/id1136220915?mt=8
You can do that with Infuse too, but it requires some networking knowledge (setting up a VPN server on your home network). Infuse can also stream movies from some cloud providers such as Dropbox.
But as far as "managing a library" is concerned, I think the client is really secondary. If you organize your files in a standard folder structure (as recommended by the Kodi project) and name them according to The Movie DB/The TV DB, either Infuse, Plex, or MrMC can scrape the metadata and build a library for you, and you can even use them interc