I'll let you in on how I decided. I have good friend (we both do IT/IS as part of our daily job) bought a Synology 1813+ in late November. His initial outlay was $1000 for the DS, then another $1200 (at the time) for 8 3Tb WD red drives. At work I have a DS 410, and I also have an old CS 406 here at home. From my experience, Synology is a bullet-proof system, but is definitely limited by the Ram/CPU in each machine. As an example, I can easily serve files and run Transmission in the 406 but if I try to add "Sickbeard" it's too much (bogs the CS down). Add to that the high (IMO) initial price and I wanted to find a less expensive way to get the DSM software working for me.
I tried a few other alternatives (e.g. FreeNAS and Ubuntu), but found that Xpenology worked great for the hardware I had, and I was able to finagle the power usage to not to be greatly different from my CS 406. It will take a long time to make up (in the difference on the electric bill) what i would have spent on a 5 disk DS, much less on what he spent on his 8 disk unit. This was the deciding factor for me, and as a bonus, an old 4-core 775 socket MB and chip I had lying around seems to have a great deal more power than his 1813... He's still trying to get Sickbeard and Plex running reliably.
I will say that Xpenology (or FreeNAS) are defiantly for someone with at least a passing knowledge of Linux (command line) and building one's own systems.
Hope this helps your decision. HD.
-who thinks that $350-400 is the cost of a /new/ DS413j...