I have quite a bit of experience with both the 4 and 6 inch AP tailwheels on my last airplane, and I also have several hundred hours with a locking tailwheel of my own design that happens to use a AP 4 inch tire on my Rocket. The 6 inch rides smoother over contrete joints and surface irregularities, but I can discern no advantage beyond that. The RV series really does not need much help to go straight anyway, so I guess it's up to you. Do you need the slight reduction in weight or increased deck angle that is offered with the smaller unit? One advantage to the AP products is that you can fit one of your choice, then simply swap out forks and tires if you want to try something different.