Should I get Undertale or Deltarune?

I want to get Undertale for 2 reasons: It's currently on sale, and I want to try out some mods. I have already beat the game on my switch, but I cannot mod Undertale on it. I also want to get Deltarune because the new chapters seem really cool! I also want to try out mods for deltarune as well. Which one should I get?