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?
I believe FNA is trying to be more loyal to the original XNA while my monogame tends to introduce new features.
I've been happy with monogame when I used it in the past. I'm pretty sure Celeste was made with FNA
FNA is for porting /supporting XNA games with minimal changes.
MonoGame is trying to evolve XNA in small ways.
For a new project I would pick MonoGame.