I have a DS209 with 2x1TB drives in it, in RAID 1, and is intended to be my backup server. I simply backup my two Windows PCs to it every so often together with my media files that are currently on the PC that runs MC. I use MC as a DLNA media server to send media to DLNA devices around the house, via an iPad controller app.
My intention is to get an additional DS411slim with a single 256GB SSD in it to start with, that will be used exclusively as media file storage, so moving the data from the MC PC.
I have been experimenting with the media server that is built into the DS209 to see if I can replace MC with it in order not to have a complete PC on all the time and could move entirely to the DS411slim. However it has limitations that I'm not prepared to live with!
Firstly, although it is better than most media servers in that you can customise your browse trees/menus, it doesn't actually offer a lot in terms of the tags that you can use. It has bog- standard Title, Artist, Genre and Album that all UPnP servers offer, and you can define your own menus containing any of these in any order and to any nesting level, but above these the only other tags available are Date, Folder and Composer, with no metadata available for Photos or Video at all (apart from date and folder). This is far too limiting for my media collection and how I want to catalogue it. In fact, it was only recently (in the last few weeks) that they actually added Album Artist to the list of music tags after months of lobbying and capaigning on their forums!
MC is infinitely flexible and configurable with regard to tagging and creating browse trees, with possibly the most comprehensive system of any media server anywhere.
Other features, of MC that I've found useful (which are not present in the Synology server) are the Analyse Audio and Replay Gain facilities, the "intelligence" that is now just starting to creep in when it comes to displaying artwork on container-level items, the discovery of artwork from net resources, and automatic thumbnail generation for videos. You might also find Zones useful in MC.