On Daf 43A, the Gemara quotes the Pasuk of the Medanim selling Yosef - seemingly, we should want the Pasuk of the MIdyanim!
The Mizrachi (on Chumash) says they are really the same, they are brothers anyway - but he did not come to answer this question (and others seem to say they are different) - why didn't the Gemara quote the Pasuk of the Midyanim?
Pesach
Rashi says clearly that those who sold him to Mitzrayim were the Midyanim. The Mizrachi merely says that the names interchange. The Targum and Targum Yonasan both translate Midanim as Midyanim. The Midyanim mentioned in verse 28 did not do anything. The Midrash clearly says that he was sold first to the Yishma'elim and then to the Socharim, and then to the Midyanim , and then to Mitzrayim.
Dov