The p3d-file is the way-to-go version of the schleichfahrt remake. You will need the Panda3d Runtime ( http://www.panda3d.org/download.php?runtime ) to start the p3d file. After starting the p3d file the runtime will download all needed data from the internet at first run. Furthermore you can simply check and install newer versions by using the bat or sh script delivered with the p3d.
The windows (full)installer is a stand alone version, heavy size and no-updating version which wont be maintained frequently.
Good luck and have fun!
P.S: currently most of the equipment has no effect. Thats known and may be fixed in later version (time is an rare ressource).