Tesla uses DSP Concepts' Audio Weaver. From the video linked below, it seems like this is another innovation that other manufacturers will follow along with gigapress, NACS, 48V, etc...Both great cars. Drive both and see which one you like better.
Tesla has better range and power but the Ioniq has better interior, Carplay, and ventilated seats. Oddly enough I ended up going with the Tesla because it had a better stereo. Found the Bose stereo to be very lacking compared to the Tesla.