undertale's definitely focused more on the "kill vs spare" rather than the actual fights (sans the genocide run)
meanwhile Deltarune says that "your choices don't matter" so there is no kill vs spare idea being emphasized, so the battles themselves are emphasized much more
I feel like that’s because Undertale’s battle system was more puzzle-like where you mainly had to figure out what interaction works best for your run while Deltarune is a s simple as picking a clear move.