I'm curious about the privacy policies behind both engines and their development. Google's gotten an advantage in many fields by simply trampling over user's rights. Apple has been more respectful of user rights, and that's kind of made life harder for them too.
New Pixels (I think 3, but it may only be 4) have an inference chip of some form. All of this is done on-device. They ported this to older devices, but it's not as fast (but still offline). The assistant is one place (probably only place) where Google has a better privacy story than most (including Apple). Notwithstanding the phone offline capabilities, you can tell the home devices (which use online transcription) "…
Having an on-device inference chip doesn't preclude them from sending back "telemetry" to improve their models. Unless they explicitly say that they don't send back data (or there's a toggle for that), my default assumption is that they do.