With a pull pull system, you don't have to worry about a pushrod bending during the push motion. A single pushrod can flex and response won't be as good as a pull-pull setup.

By servo wheel, do you mean a wheel on the rudder end that is taken off of a servo? If so, the reason for this is so that the width of your connection on the rudder end is the same width as the connection on the servo end. This makes for the best setup because the geometry is simple and you'll always have a perfect parallelogram, no matter what position the rudder is in.

-Eli