I like the paper money so you can keep the amount secret.
Or they could have single-sided card money like Chinatown. Or worst than Chinatown but way better than regular paper they could have that linen-type paper money like in St. Petersburg.
Personally I think the Chinatown money is the best; medium heavy card stock with a good coating.
The card stock / lamination Flying Frog uses would be great money too.
