It's common to invoice students based on the Hobbs meter while service intervals depend on the Tach time (which relates to engine revolutions).
When entering flight details, you normally record the engine hour meter. For aircraft with a Tach timer fitted, this should be used because it determines the next service time. Otherwise, for aircraft fitted with a conventional Hobbs meter, use that. If an aircraft has both, use the Tach timer.
Billing in this system is based on the flight start and end times by default, so are equivalent to the Hobbs meter but with better resolution.