Considering the regulatory view of why a standby altimeter would be required, then the use of the same pressure datum as the main altimeters would be more appropriate.
If both main altimeters (ADCs) fail then the standby is already set to the datum in use; alternatively if the standby system is required to resolve an ambiguity between the main altimeters (comparator alert) then the pressure datum in use enables a quick check.
Resetting the standby altimeter also provides an additional error check as one pilot is required to set the pressure datum twice, which should require a cross check with both main altimeters.