i have 2 sentences as follows:
민수 씨는 이번 시험에 1등을 했다지요?
and
민수 씨는 이번 시험에 1등을 했다면서요?
I wonder how these two structures are different? Can you help me to understand them? Thank you!
They are about the same in your examples. I think 했다면서요 sounds slightly more informal, but the difference is negligible.
One minor thing is that -다면서요 has another usage while -다지요 (short for -다고 하지요) is used only in this one sense (but it can be used in both declarative and interrogative sense).
Here, 다면서요 means that he went out saying he was going on a school department trip. This is different from your example which means "I heard them (some people) saying that ...".