Undertale Yellow is a prologue to Undertale, featuring the human with the yellow soul, Clover. It's got a new cast of characters and new locations, and features a Flowey that's more friendly than usual.
I think Undertale's slighty better. It's a bit annoying to have to input moves for three characters, especiqlly at times when you just want to spare one enemy. The fact that Deltarune is so close to Undertale in terms to quality considering how bigger of a game it is very impressive. DR Chapter 1 is rough though, I'll say that. Very mid.
Don't worry, Undertale/Deltarune will remain ludicrously popular without you and millions of people have played the Undertale-worth of free game that is already out there.
