The Cheiftain has a bigger payload and can have upto 10 seats!
Navajo can have upto 8 seats.
Both are excellent mini airliners and handle surprising well.
Performance is similar, its a few years back but I think we used to fly the Cheiftain around at 160 knots being very kind, 180 knots achived by using lots more fuel! Sure you can google a flight manual up!
You will find the operating cost the same, the purchase price will obviously be higher for a late Cheiftain than an older Navajo.
One of the best aircraft around, imho, much better than the effort from Cessna
