I've played both now, and I didn't notice much of a difference in the training levels.

Thanks for the info, but if you check the stickied threads at the top of this forum you'll notice the same information is already available and in further detail. :)

FSOpen is already well loved here. But more importantly, the game ENGINE is what's open source. FSOpen does not distribute the commercial game files (anymore). It even says right at the top of Turey's webpage, which you linked to, that the installer doesn't include the retail files, and he links here as a suggestion on where to get them.
They are necessary if one wants to play the Freespace campaign. They're only not necessary if one only wants to play full conversions or other mods that don't use retail resources.

Might want to check in your FS Open launcher if everything is checked under video and features. Also you probably won't notice that big of a difference until you go into your first mission and see all the pretty reflective surfaces on the cargo transports.

See, for me, the best part is the game content. :D If the engine wasn't as awesome as it is, I wouldn't have gotten into it, but the content is what holds me to this game so strongly! THANK YOU for providing it, GOG.com, I just spent some Christmas money on it. :D