To charge Hobbs time is probably a rip-off, and leads to pilot frustration.
Excuse me?
You pay for the use of the aircraft, not per hour flight time.
You don't pay per mile driven on a rental either now do you?
You get charged per day or portion of a day that you have the car in your posession, why would it need to be different for an aircraft?
You still pay for the rental car even if you've been stuck in traffic all day right? You don't go argue at the AVIS desk that you've only driven 30 miles in 3 days because of traffic jams?
For commercial enterprises eg flight schools the aircraft is a source of revenue.
For a small group (4-5) you may decide to charge on tach time.
Hobbs time usually ends up being 1.2 tach or 1.3 tach if the aircraft is flown correctly. You can always tell if somebody has flown it on "rental" power
So your choice:
$100/hr for Hobbs time
$125/hr for Tach time
The chicken or the egg my friend....