First times errors will be less costly and easier to repair with the Easy Star.

The number of channels is irrelevant.
Pioneers of radio control hardly had one or two channels available.

In the Easy Star there is an intentional coupling between yaw (commanded by the rudder) and roll, which is what starts and facilitates turns and horizontal circles.
Hence, your rudder's servo should be connected to the right stick of the transmitter, as explained above.

For the PTS, that yaw-roll coupling is less pronounced, and the roll is mainly induced by the ailerons (that the Easy Star doesn't have), while the yaw is induced by the fourth channel (rudder-left stick of the transmitter).

Yaw control is important for planes with landing gear that have to land in a straight line under cross wind.

Since the Easy Star has no landing gear, it can belly-land on grass in any direction from which the wind is blowing; hence, yaw control is not really necessary.

Check these links out:

http://www.grc.nasa.gov/WWW/K-12/airplane/elv.html

http://www.grc.nasa.gov/WWW/K-12/airplane/rud.html

http://www.grc.nasa.gov/WWW/K-12/airplane/roll.html