Overall, besides age and minor things like a nose baggage compartment, etc. the big difference is in some of the systems. The older Commanches had generators instead of alternators, although this may have been replaced by STC. They also have a much different gear system. The Commanche has an electric motor that drives a big jack screw that is under the floor between the front seats. There is a 'trolley' that runs back and forth on the screw that is connected to the gear and brings it up and down. The gear is held in position by 'bungees' that have to be replaced every three years. They function like an overcenter lock.
There is the familiar mechanical flap lever between the seats, a trim crank on the ceiling (takes some getting used to), and another large handle with a red knob like a gear shifter on the floor. The 'gear shifter' is for manual gear extension. It is either lying folded back on the floor or straight up and down depending on the gear positon. DO NOT PUT ANYTHING BETWEEN THE FRONT SEATS that will interfere with the movement of this lever (warn the passengers so they do not squish their feet).
Depending on the year, there may be other quirks like a switch to turn on the turn & slip indicator. Commanches also have rubber bladder tanks, so watch for blue staining on the back of the wing and wing root during preflight that may indicate a deteriorated bladder.