Soundtrack

the emotional and fitting soundtrack for every scene, where almost every different part of the game has it's own soundtrack, meanwhile deltarune has a weird funky soundtrack that is the same, almost everywhere, the soundtrack just doesn't fit with the bosses and the place

Bosses

The bosses in undertale actually have a different goal, toriel doesn't want to lose you, papyrus wants to catch you and give you to undyne, undyne wants to kill you for your soul, mtt doesn't really have a goal she just wants fame, sans wants to kill you for killing his brother, asgore wants your soul, flowey just wants to torture you, meanwhile deltarune bosses don't have a SINGLE GOAL except kill you, for what? FOR NO REASON

Zones

In undertale, every single zone is special and unique! the entrance, the ruins, snowdin forest, snowdin town, waterfall, hotland, new home, meanwhile deltarune doesn't have as much variation

Battle system

In undertale, toby and the other creators actually took time to make new sprites, and the fights are actually tricky, with the attack system having to be exact (same as in deltarune) and it just is more interesting, meanwhile in deltarune it's pretty weird, you get a currency for almost not dodging attacks when the whole point is to dodge them

Other

Undertale has actually good gameplay and it doesn't seem rushed or low budget at all, deltarune is just a rushed mess

that's all, thank you for reading this