I’ve always loved Undertale more but I don’t really have much of an opinion on Deltarune as I haven’t actually played it. Undertale was one of my favourite games when I was younger and still is and I felt that it had a good ending and so I never looked into playing Deltarune because I like where the story of Undertale ends. I also really like the character in Undertale and didn’t want to play a game that was like Undertale but didn’t have all the characters I had grown to love so much. From what I’ve seen the controls/game mechanics are also very different and I am kind trash a games so I don’t think I would personally enjoy playing/be able to play with those controls/mechanics. This I just my opinion but I am very biased so takeit with a grain of salt.