The next version of SkyDemon will use great circles when you plan a new route. Previous versions have always used rhumb lines.
Unless you're planning journeys with exceptionally long legs (hundreds of miles), this will likely make no difference to you. For interest, though: for the same leg from A to B, a great circle will have a shorter distance and a rhumb line will have a steady track over the ground. If you're using a GPS to navigate with (as most now are), great circles make more sense.
Routes saved from previous versions of SkyDemon will continue to use rhumb lines, so you'll see no change when you load a route you've carefully planned to avoid airspace and other features.
Long great circle legs are identifiable by their curved appearance on a Mercator projected map.