I started out with the 3/4 length, and then added the pieces to make it a full length because I needed more length/room. I also replaced the wind deflector with their under-mount light bar. Had to add a full length of rubber door stop, between the top of the light and rack, and that dealt with the wind noise issue.
Obviously, I can now change between either size rack (with a certain amount of labor) if I need too.