I have the DS413+ , just got it all nicely setup to stream wirelessly to a WD TV Live box. Currently 'only' have 3x 3Tb WD Red drives in it, as with the hybrid raid system I can later add a fourth drive of higher capacity (4Tb are available now, and by the time I need a fourth... Who knows... And then you can replace the 3Tb drive one at a time with the future 6Tb drives no doubt...).
I'd say the + is well worth it, if you need more processing power for transcoding video on the fly.... Especially useful for Apple TV use unless you're obsessive about your media formats and don't need to transcode mkv, dts audio etc etc etc. Also means it'll run the extra applications more easily... And there are many good ones - search around and add the Synology Community apps to the default choice.
http://www.synocommunity.com/
I did find the built in Syno Videostation to be pretty good - both for collecting all the metadata for movies and TV series, and ease of use with the AppleTV. Much nicer to browse your collection on an ipad, then airplay it to your TV - but for me the lack of subtitle support is no good at all... And NOTE this is an AppleTV issue, as it incredibly doesn't support subtitles over airplay!
tried using Plex, and a Plexconnect non jailbreak hack to get it working on the AppleTV, worked but the Plex wouldn't see many,many of my series and refused to catalogue them, even though the same shows would be picked up ok if run through Plex on a Mac laptop.... Sigh.
anyway, that Plex hack could be obsoleted by an Apple update... So ended up with
The Western Digital TV Live, supports vast range of video formats, subtitles are fine, and I prefer to just use an NFS Linux network share to the Synology and use an app on the Synology to catalogue everything etc.
The Synology Download station Torrent client works great, there are others available including Transmission.
I'm mostly using SabNZB and Sickbeard to run things on the NAS - searches for latest tv shows I'm following, downloads them and even gets the subtitle I want, renames the and puts them in their proper folders etc... All auto-magically.
I was looking for Drobo NAS, but in Thailand they are hard to find - glad I need up with the synology, the available apps and the community support is great.