Tourist/Visitor-Charlie Card or Paper Ticket?!

Hey all, I'm excited to visit your beautiful city, just had a question about the metro (or T I think you call it)

Does it make a difference, if I want to buy a 24 hour pass, if I get it on a plastic Charlie card (given that I can find one, I have read you guys have trouble finding them?!) or does it make a difference if it's just a paper ticket?

I hope I'm not asking too dumb of a question- any help/guidance is appreciated :)