UE5 if you have a beefy computer. Doesn't have as many quests as the UE4 version, but is the currently being updated version that has a couple of mechanics (like pregnancy to a degree) that was supposed to be in the UE4 version.
Go with UE4 if you don't have a beefy computer or don't care about new content, UE4 has more quests and scenes that aren't in the UE5 build. As it stands the UE4 version is finished as they left it, it has been abandoned, UE5 has less features and story quests, but I've personally been having fun with both.
Now to ask a question that someone can hopefully answer, when is the update for the UE5 version? I understand that there is a version for Subscribestar/Patreon and a Steam version, when does the page for this game update, when we get the Steam update or is the current downloadable version of Carnal Instinct the newest available version?