I'm on an asustor as20something TE. It works but I think its a low end model so opening files is kinda slow. But its been running 3 or 4 years np. So there's that.
So I've been researching for an upgrade. And will probably get a Synology. Between Synology and qnap, Synology has better software, qnap has better hardware.

Suggest you go for a ds920+ (just announced). Has all the specs and support for everything up to VM. So will be overkill specs for filesharing. I dunno, I need to make sure I dun get anything slow or laggy. Use 2bay first and add hdd later when you need. Synology hybrid raid will allow adding hdd of different sizes in the future.
Cheaper option is ds420+ with the same software and shr but no esata extendability (+5bays) that the 920+ has. Also a good option. But the present 418 and 918 pricing not very far.
Dunno much about qnap. Stopped looking after the reviewer said Synology is the Apple of NAS. I just want something I set up once and it just works.

Yeah there's no real wrong choice outta the 3 brands. So many options. I'm picking up the Synology mainly for the iphone photos auto back up. so I need it to run reliably, automatically without me having to monitor it. And lag free. I will overkill to minimise lag.
Good luck on your hunt.