It is the same game just darker story... the og game is less buggy . revamp needs more fixes for it to be less buggy... the og game has most if not all book of faces quests done only the mafia quests are not done in ...revamp is missing some of mafia's quests also I think there is two mafia quests you can do that's it ... not sure in og game about mafia quests I think there was one quest you could do i think... depending on how may hour you play the game both games have around 10 to 30 hours of play time but becareful of the bugs some you can fix some not so much...
What really ticked me off about OG getting Revamped was the original version was finally starting to get to the Lucy content just before BOTN decided to do the revamp to fix the original bugs that were in the game as well as improve the storyline. Irony of it all was that as you pointed out as the game went on it ended up become even more buggy than the original to the point where SOTN needed to make bug fix patches to make it playable.
To be honest though a lot of the OG players who played the OVA version of this were so dedicated to this game that we found ways to get around these bugs as a team myself included seeing as on my end I literally saved the older versions of the game to help other stuck at certain bugs bypass them.
Another thing that didn't make too much sense to me was the also the fact that for some odd reason BOTN decided late into the game he wanted to add Netorare for the MC which with the way the game was made would be just about impossible to do. I mean MC was literally fucking everything and anything that moves, with him being a Gigachad Dwarf it made absolutely 0 sense to do this.