1The beginning
http:// at the start is normal for onion addresses; the Tor connection itself provides the encryption.
2The onion name
The 56 characters before .onion are the identity of the service. This is the part to compare.
3Anything after the slash
Paths like /login point to pages within the site. They can be changed freely and say nothing about who runs it.
4Practical rule
When two torwatch URLs differ, check whether the difference is in the onion name. If it is, they are different sites.