UserManager is RADIUS, all version of it.
The difference are from features in every version, how they work. v7 more advance than v6, one of the new feature that v6 dont have is 'attributes' which is really help us to setting many things.
But the problem is v7 is not final yet. As u can see at help documentation at Mikrotik Official
https://help.mikrotik.com/docs/display/ROS/User+Manager some section still not provided. It's there in usermanager v7 but as far as I can test, some setting still not working, even I try to match it with v6 settings that works before (or maybe I dont get how to make it working
)
About v7 webgui, as told in documentation (previous link) Webgui still there but just for user to see their profile. All administration command directly from winbox gui or via terminal. I see it as nice upgrade and more secure implementation.