님 is much more formal, and is more commonly used with titles rather than names. 선생님 (teacher), 회장님 (chairman), 하나님 (God). It can be used with names, but usually after someone's full name and is extremely praising. On the internet, kids will address people with 님 just by itself if they wanna be formal but that individual is anonymous or whatever.
You'll mostly see 씨 at the end of names. It's formal, not super formal but not casual. It's how you'd address someone you're not close to, but who's on the same 'level' as you or below, socially.
Societal norms are changing super quickly in Korea and a lot of lines are being blurred, but this is a general consensus I'd say.