Hey, so I use both right now for my PhD research. Happy to try and answer any questions about the two devices.
Short version is: they each have different pros and cons (beyond the inclusion of a very convenient strap ), but very much depends on your intended use case.
Moxy is well established, tried and true, more robust (physically, e.g. waterproof. And in terms of existing data ecosystem, e.g. garmin IQ apps, compatible software, etc.) and arguably simpler (it spits out a signal with no added interpretation). But it is older hardware at this point with certain signal quality limitations and low-ish sampling rate (0.5 Hz or a noisy 2 Hz).
Train.Red is new, very promising, very high signal quality, much higher sample rate (research device is up to 100 Hz, I think consumer grade is 20 Hz? More than enough), and has a developing app with autoregulation feedback. Cons are it’s new and not proven (in terms of durability, software/hardware bugs, commercial viability), the app front end adds complexity for promising but unvalidated interpretation.
I posted some figures from some of my current pilot testing using Train.Red if you want to get a sense of the signal. I have some combined datasets w/ Moxy & Train.Red that I might be able to post.