Terastation, QNap TS, Zyxel or Synology?

I'm building a NAS system for a small office with the following requirements:

• 2x 2TB NAS. One located in Italy, the other in UK.
• Live full 2-directional sync/replication between them.
• Easy user access both from within the LAN (on either site) and over the internet using windows explorer and/or mac filesystem.
• Easy user access both from within the LAN (on either site) and over the internet for iOS.
• Remote admin capability.
• CCTV surveillance connectivity and storage.

As always main metrics are price and upload/download speed. I'm looking at Buffalo Terastation, Qnap TS series, Zyxel and Synology Diskstation. Has anybody with expreience of these got any advice or recommendations?

Thanks