Nexus is Google's line of flagship or developer devices. So they come with a pure, unmodified version of Android, the way Google wants it. So they always get the latest software updates first.
The Lumina is a Windows phone, I don't know too much about them, but Windows Phone seems to be a pretty decent OS, but I'd still take Android over it. Nokia makes excellent hardware, IMO they make the best mobile hardware, just too bad they decided to go with Windows.
Rooting just pretty much takes the child safety off your phone so that you can modify the OS and anyway you want.