In brief, every solution has its' own advantages and disadvantages. For example,
Windows solution has no basic functionality to operate with directories, and as
result you need other way to manage your resources effectively.
In other side, all solutions except WinQuota are not providing accurate
hardware limits. So, SpaceGuard SRM uses directory rights to manage quotas
and you'll get conflicts with your policy being used at the same time with
quota management. WinQuota uses other algorithms to track quota limits; it
provides accurate byte-n-byte limitations for all file activity without
any changes of other system and security settings.
Other notice abilities like email, pager, SMS and other are not required for automated
quota management. Really, it is necessary for SpaceGuard and others only as it have
no advanced WinQuota techniques; and every quota overburn requires
system administrator invasion to solve conflicts.
In brief, WinQuota is alone system with all basic functionality can operate
without system administrator continuous assistance; the all attempts to reach
over quota limits will fail at the system level, and the user' application
will be notified via standard system error codes. The hard quota limit would
not be exhausted in all kind of operations, including file moving, renaming,
copying and other.
The comparison detals in table form is available here.