Hobbs doesn't necessarily align with engine start & stop. It depends on the way the switch is wired. Variations I've come across:
* Oil pressure
* Gear up/down
* Squat switch (weight on wheels)
* Airspeed switch
* Master on
I log using decimal time in my log book, noting off & on blocks times, and airborne & landing times. One for my log, and one for the aeroplane. If the plane has a Hobbs (and its maintenance is based on it) then that's what I record for aircraft flight time.