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.