Keep in mind there's a lot of other factors. If what you need is a simple file storage system, using one of these NAS devices is the way to go.
No real maintenance or patching to worry about
Doesn't suck down nearly the power that a full PC does
Uses very little space
More easily portable
Rebooting one PC (or opening up a VPN, as I frequently do) doesn't affect the central file store
Very little noise and heat generated vs a normal PC.
The difference would be, if you needed other functions (like a web server, print server, mail client, etc.) that the PC could otherwise take care of. But if all you need is a simple file share, a NAS is the way to go.
- G