Investigated file size limits, and my conclusion is that stoat has a 20 MB file size limit, and fluxer's is 25 MB. 🤷♂️ I don't know about character limits, and I don't need to know because I never post long messages because I don't treat this app as a bulletin board.
You seem pretty set though, so enjoy your freedom to choose whichever platform you like, that is subjectively better for you. 🙂
Just found out Fluxer intends to add federation soon as well, which seems like a one-up on other platforms? I don't think Stout has that unless I'm misunderstanding what Stout is built on.