The Microserver doesn't have any USB3.0 support out of the box.
Windows 8.1 is possible (it's running on mine), but at the moment you have to disable the on board NIC and add your own. Windows 8 didn't suffer from this problem. I'm not sure if/when there'll be a fix available.
The main advantage is that they support four 3.5" drives, and with a bit of work they'll support six.
That Acer looks fine, but only appears to support a single drive (or two if you remove the optical). If that's all you need I'd stick with the WDC My Cloud you mentioned originally.