Does the ring still need to be recharged? That's its main drawback. Also I bet nearly all of us would be very very low end GLs and none of us could even lift the hammer. So I guess being a crappy GL is better than having a hammer I couldn't lift.
In general Mjolnir is more powerful. But these two weapons heavily depend on who is using them.
Shouldn't use feats from Thor. It's hard to tell what are Thor's power's and what are the Hammer's powers. Even Beta Ray was already able to match Thor -before- he had the hammer.
The Team Fantastic has decided that the JLA is too dangerous to be allowed to remain on earth. They plan to capture them and put them into an alternate earth that never developed intelligent life. They have access to all of their own inventions and resources but no time travel (but there is time manipulation if they can pull it off), ultimate nullifier, reality gems, or cosmic cube to start. If the Team Fantastic can build or get such in the time allowed then they can use them. Team Fantastic also has Black Panther, Storm, and She Hulk as they were once members.
These are the New 52 versions of the JLA. Hulk is Green Scar to start. Banner is in Banner form to help with prep. The battle takes place in Metropolis where the Team Fantastic has lured the Justice League with a false alarm. How much prep time do they need to win? Morals on.