Eternum has some pretty good humor in it too
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.