To the user they're broadly equivalent. For a few versions ConnMan was vastly faster than NM at IPv4 address resolution (for reasons I never bothered to puzzle out -- all I know is that MeeGo development boards would come up on the same network 4x faster than my laptop). ConnMan is smaller. It has a much (!) cleaner D-Bus API, though it integrates poorly with all the legacy scripting in existing distros (whether that's a feature or not is a flame war I'd rather not get into).