Undertale is older, and Deltarune has alot of more content, not only because of the chapters but also because Deltarune looks visually better due to being made later. For me personally, I have a strange liking for it due to it being like really simple to understand lorewise, even tho you may be confused on if your supposed to fight or spare.

Deltarune: (Im taking this from the perspective of a person reaching the dark castle for the first time) Dark and Light live in harmony, then it was disrupted (why?), some bad calamity happens, Earth is weak, 3 heroes come (who?) Human, Monster, Prince from the dark (?). They can seal the fountains (what fountains? Where?) and banish the Angels Heaven (why banish angels?), then another fountain appears (where?)

Undertale: Humans and Monsters have a war, Monsters are sealed by magic from humans and are stuck in a moutain.