There are outfits out there who will charge on the hobbs, unless the tacho is higher, when they will charge on the tacho.
So if you spend most of the time in the pattern or practice area you will pay on the hobbs - even though expensive stuff like engine maintanance is done of the tacho. If you go on a long cross country and operate at high RPM you could find yourself with a higher tacho and pay more than the hours you actually operated the engine.
Just something else to watch out for
