Great question: the free version basically has all of the similar long blue-red hangars and aircraft parking aprons hand-modelled plus added stock models here and there and many small fixes like lines, taxiways, aprons, unnatural grass texture, etc – mostly useful to people familiar with the airport and just wanting the most glaring problems and default messy scenery fixed. Pro version has most of the interesting stuff: hand-modeled main building, WWI museum buildings with rolled-out aircraft (including Fokker triplane), entrance sign, animated true-to-wind waving flags, removed ugly default beacon tower with authentic 3D beacon model on the roof ot eh main building, correctly placed lighted windsock etc.
So you may want to try the free version if you like, so the airport is more recognizable from the air, and to check that you like the quality of the models (although they are simple models in the free version), that it doesn’t affect your FPS negatively etc. and then consider the Pro version ,