Acta est Fabula (AeF): Original version, 18+
Amantes amentes (AA): PSP port, all ages, adds extra strories and a true route
Acta est Fabula HD edition: adds the same interface as AA, no other changes, can't be hooked as far as I'm aware
One more thing to note: Bei sprite is floating in AeF in 4:3 resolution which is a minor annoyance (you can swap the aspect ratio with F3 in the HD version)
Here are the 2 guides:
https://seiya-saiga.com/game/light/dies_hdaf.html
https://seiya-saiga.com/game/light/dies_hdaa.html
First, you start with AeF and read the first 3 routes in this order: Kasumi -> Kei -> Mari (they are exactly the same in AA minus h scenes)
After finishing each route in AeF, you switch to AA and skip it all the way through, open the Other Story menu, and read all of the extra stories that are getting opened (they are very important for a better understanding of the story)
THEN
As you can see in the guide, AeF has an additional Rusalka side route, which is exactly the same as Rea, but adds several different scenes with Rusalka, and an important scene after the Rea ending
BUT, AA has combined Rea + Rusalka in 1 (in favor of Rusalka) and it overall makes much more sense than AeF. But on the contrary, it removes several important Rea scenes which makes their romance much less impactful in AA
So here is what I THINK is the best possible way to get the most out of it:
1. You start Rea in AeF and read it up until the last Rea H scene (which comes after the last choice in the AeF guide)
2. You put it on hold and restart it again while now following the Rusalka route in AeF and collect all the h scenes, skip to the moment where you stopped previously, and close the game
3. You swap to AA, skip to where you stopped in AeF, and continue reading it while following the AA guide
WARNING: Where there is supposed to be a 2nd Rusalka h scene in AeF, it is replaced by a different scene in AA so make sure to not skip it (by the way the first h scene was kept as it is just with the CG being cropped)
4. Continue reading and finish the routes in order (Rea1 -> Rea2 in the guide). This way you will get all Rea scenes, all Rusalka scenes, and everything in a logical order
Here, save it somewhere and refer to it once you get to the Rea route