The longer version was the same as the ABC (American) TV version with two differences-
The TV showing was horribly dark. The VHS version corrected that. Also, when Epsilon Nine is monitoring the fight between V'ger and the Klingons, their computer shows the fight on one screen, on the other a printout of the computer's spoken words. An old 4x3 TV picture could only show one screen. ABC showed the computer's words, the home video release showed the screen with the Klingons.
As for "stuff from the cutting room floor" and "unfinished sets", in one extended scene the latter is true...if you notice it. It doesn't stand out (in my opinion). The rest of the extended scenes are screen-worthy in every way. They just make for alternate presentations of scenes.
I never realized the longer version has never come out on DVD. After it was released on VHS it became the only version you could buy. Now the opposite is true? Weird.