I think that if you have hard racks, the 9'6" is easy to transport without too much drama.  If you have soft racks, there's a bunch of irritating things that can happen.  One of them is that the straps tend to vibrate in the wind and create weird noises as they hit the car roof at high speeds.  If you're worried about the buckle slipping, you can tie a half-hitch knot on the loose end to another part of the soft rack.

Another consideration is whether the trunk or hatch of the car is easily accessible with the board strapped to the top.  BTW, do you have a Subaru Forrester now and can you fit a 9'6" in the car?  With passengers?