I suggest thinking about what kind of sorting/filtering you plan, and then making sure you understand how that functionality will work on a large set of messages.
With a list field, my understanding is that if you want to filter it, that is happening client side, meaning Bubble is downloading lots of records (in the background) to the browser and then filtering it there to find enough results to fill your repeating group. In that situation I would worry about discussions where thousands of messages are being downloaded to a user’s phone, which would be a slow and laggy experience.
If you linked the other way (each message is connected to a discussion, a many-to-one relationship), the filtering can happen server side via a regular “do a search for” (very fast) and it will only send to the browser the results.
(PS: If you haven’t yet read this book for info on how Bubble works behind the scenes, I highly recommend it: The Ultimate Guide to Bubble Performance - the new edition is out (now 210 pages!))