Synology Diskstation DS216play is the latest 2 bay NAS designed for home users and especially as a media server.
Synology Diskstation DS416 is the latest 4 bay NAS designed for home users or small companies. It doesn’t have all the features that the improved DS415+ have, but are still an advanced NAS made for most users.
Synology Diskstation DS1515+ is a high-end NAS with most of the features Synology can give you and designed for advanced home users or small businesses. DS1515+ is running DSM that is an OS with lots of extra features beyond the standard NAS. What is even better is if you don’t want to use all the extra features, you don’t need to install and use them.