I like both, however, it seems to me that Medallion is going to be trashed for Gold 95% of the times (i.e., everytime you do want a Gold, which for some engines you may not want).
Using Medallion to trash Coppers seems pretty awful (even worse than Loan, which is already ok but not great, and costs $3, doing that with a $5 would be extremely slow). Medallion may seem nice to deal with transition treasures or even actions (Silver in engine or Colony games, Potions that you don't need anymore, cursers when Curses are out), but those cases are not that common and you probably don't have lots of transition treasures anyway, so its hard to make them collide with yout Medallion.
Its really similar to Upgrade, worse for engines, better for BM. But in BM, you would most likely trash it for Gold at the first chance. So, how different would it be from "worth $1, trash this, gain a Gold"? Probably not a lot.
EDIT: Maybe "gain a $6 cost" as you would probably take GM if its on the board, and maybe also Goons, over Gold.