There are three considerations;
1. Storage and back up.
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.