This question has been asked before. In general, they're two different sounders with similar capabilities. There's more versatility with the TZTouch 3 internal sounder, being able to use both conventional CW transducers as well as CHIRP, and with the option of the DI-FFAMP to use more powerful CHIRP transducers (e.g. R109LH). However with a handful of transducers (e.g. B275LHW), the DFF1-UHD offers slightly more features that just haven't come to the TZT3 yet (e.g. Bottom Discrimination or Accu-fish).
