Figuring Out the TMoney Card

I am heading to Korea this week and have already bought the discover soul pass, which acts as a built in tmoney card. However, I heard about the M pas and was wondering if there is a difference or would I save more by buying an M pass. And if not, how much money should I put into the Tmoney card if I dont get the M pass. Also going to Jeju so wondering if either work there, thanks!