Tbh I prefer Undertale over Deltarune, so far anyway, that could change as more chapters come in but.

For one, I prefer Undertales soundtrack, while Deltarune has good songs (most of which are from part 1, part 2s soundtrack hella lacking) it doesn’t have as good songs as Undertale.

I won’t comment on the story since Deltarunes isn’t finished.

I much prefer the battle mechanics of Undertale over Deltarune, plus the games battle UI and style.

Finally, Undertale has much more overall charm than Deltarune has so far

Granted all these opinions could change in later chapters