I like both TBH
Undertale has a great story and characters, as well as a lot of moments that make it a great game. (My favorite one is when you're hiding behind the convenient lamp.) In fact, it's been two years, and I haven't even considered leaving its fandom.
Deltarune, on the other hand, has a new concept which tells us what would happen if the monster and human war never happened. Even though it's not done, it does have potential to be just as great as Undertale.
I don't care if I get canceled for this, I love both games!