Basically the same. “There’s not a lot of guys around so you need to fuck ’em girls”. And then they proceed to fuck ’em. Borderline hentai.
The story in Shuumatsu no Harem is a bit more serious, if I might say that, while Parallel Paradise is pure fantasy (with very small signs of Okamoto’s bullshit, but I think it’s safe for now).
Both have this apocalyptic world where people died/disappeared and also have extreme ecchi scenes that will surely make your nose bleed. Although Saikin focuses more on girls and how they behave (more specifically the main heroine). Shuumatsu is the opposite and focuses more on guys. Both are AWESOME with great story line and good character development.
One man having multiple relations with many partners. Even though the two stories are set at very different time periods both main characters find themselves waking up at different periods of time trying to grasp the dyer situations they are in. Both mangaka's show amazing display of artwork, unveiling some sensual scenes between the characters.