I was trying to compare the two. At first glance, MonoGame has far more stars and recent commits. Or is it just in maintenance mode?
MonoGame is trying to evolve XNA in small ways.
For a new project I would pick MonoGame.
I've been happy with monogame when I used it in the past. I'm pretty sure Celeste was made with FNA
https://www.pcgamingwiki.com/wiki/Celeste https://celeste.ink/wiki/Version_history
Monogame on PC used to be somewhat buggy in my hobbyist experience.