The Venture is a nice sled with a very comfortable back seat. The AC is going to be a lot slower, due to the less horsepower. What do they want to do? Economocially sightsee at a cruiser pace through the woods or have a little more performance. The 4 stroke does not burn oil, and burns less gas, so all else being equal, it should cost less to run. It also wont make their cloths smell like oil and gas. If dad wants a little performance and speed when going alone, get the Yamaha. For just putting along, I would get the AC.
I want to ride every day, all day, as fast as I can.