Played both of them! Harem Hotel is a great AVN without a doubt! Somewhat similar looking characters with Once In A Lifetime but very different and different unique stories (Caribdis' previous game before Eternum is Once In a Lifetime)
But Eternum? No offense to other AVNs but THIS is without a doubt one of the best AVNs you'll come across, it's really rare to come across an AVN with great plot, great characters, great progression, and most importantly, frequent updates.
And oh boy, Eternum checks out and MANY MORE! And even the design of the characters was a massive improvement compared to Once In a Lifetime and Harem Hotel! It's so unique!
I promise, you really won't regret playing Eternum^^