Nvim is more optimised, while emacs is more extensible. Basically you can modify core parts of emacs while it runs. I tend to use both, depending on the situation, with a lighter nvim config. Sometimes the 3 second emacs startup time is annoying so I use vim then. I think its fine to try both.
Regarding emacs declining popularity, I think that in the long term it could be a problem, since most people don’t want to learn elisp just to configure their editor. Elisp is very powerful in emacs, but its design is very different to other languages, so as emacs contributors get older, it could possibly lead to less and less new contributors.
Idk about the vim distros, but I think Doom Emacs is easier for beginners to get into.