Vacuum Cleaners Discussions

I am trying to help my son's preschool (we're a co-op) select the best vacuum.  Everyday lots and lots and lots of sand makes its way into our classroom.   The sand has laid to rest an expensive Oreck in less than a year and a lower priced bagless clocks its demise at 18 months.  I lent them my miele canister for a couple of days to see if the teachers like it.  Is a canister the best and if so which one?  Any suggestions would be greatly appreciated.