While comparing both games seems fair due to their quality (visuals and writing), they also aim to be two different things.
Eternum aims to be a more mature and complex AVN, as great as any big developers' visual novel, with top-of-its-kind rendering, a high quality and tasteful storytelling, and an infinite-possibility universe. Although sprinkled with comedic relief, mostly from a funny sidekick, the main theme is serious and erotism has a slow build up.
On the other hand, in my view, Realm Invader aims at a larger, more laid back audience that expects comedic relief every other scene; something that can be enjoyed with a little less involvement in the long-term story. It has more action even in normal conversations, a more "aware" MC, earlier/more achievable romantic development with other characters.
They both fill their respective place in the AVN world gracefully, although Realm Invader deserves WAY more attention than it current has.