Hello,
I'd say it has to be the objective measurement as I'm a recent contact lenses convert and my glasses had a stronger prescription (just) due to the lenses being that bit further away from the eye (0.25, 0.5) - with my contacts it's 0.00 and 0.25.
Good question though - I seem to be remember a phrase something like "correction must not exceed blah blah blah" on the CAA's website indicating that if you just fell outside of limits with spectacles (i.e. they corrected too much), you might get away with contacts (less correction required). This wouldn't be a measure of objective error though, only a measure of how much correction you need with a certain device...
Not sure about this one...
V1R