Mimosas with champagne or prosecco?

Help me settle a debate – I went to a friend’s house where they had mimosas with champagne and was told champagne is the only option for mimosas while prosecco doesn’t work, even though they had spent the same amount on the champagne as they would have prosecco. Not sure if I’m just clueless, but I can’t tell the difference when it’s in a mimosa. Does it actually matter??