Both are really decent, my friend has a Q-Nap and likes it, I almost got a Synology DS413j (before getting an HP Microserver for its added functionality and better price)
Either is a great choice, personally I'd go for the Synology, as its just for Docs storage etc... you probably don't need the extra CPU power in the Qnap or the dual Nic's etc.. which is where the price difference is.
Well for the few extra bucks (60€) is the extra CPU power and dual NICs worth it?
Now that you mention it, at my workplace we have a Q-NAP as well (admind by me) and for the most part, Im pretty happy with it. I just made a VPN server with it as well and it works pretty good
Im happy with a Q-NAP at my office but Ive heard a lot of good things about Synology.
Also, viewing other models, should I go with ARM or x86?