There are three considerations;
1. Storage and back up.
2. Performance.
3. User interface.
If all you want to do is make content stored on your PC available to DLNA compatible media players around the house then a basic NAS is all you need, it provides both backup to your PC and media server function in one. WD Live NAS products are great for this.
If you have a large media library and need to ‘host’ this on a NAS then you need something with two or more bays and RAID to provide at least an element of backup protection. I say element as there is backup and backup, some will argue quite rightly that a backup is something copied and stored separately, even off site. Anyway, a WD or Synology multi-bay NAS will provide this function.
Then there is performance and user interface. Synology have become very popular due to a great user interface, lots of features and great read/write data speeds making them very easy to live with. If you buy cheap you’ll soon discover why,
Set out your requirements and the right product will present it’s self.