I guess I'll start here, since it involves 20" wheels. I'm asking on behalf of a coworker who bought an Origami Crane (in part with me and my Brompton as inspiration ) for commuting. She said she gets a lot of flats, and wants to put mountain bike tires on it, which struck me as a bad idea, as they would slow her down on city streets. I don't have a lot of experience myself however, and don't quite know what else to suggest. I know Schwalbe Marathon and Marathon Plus come in the right size, but I suspect they may be pricey for her.
So, what sort of 20", 47-406 tires might be puncture resistant and suitable for city riding? Cheaper is better. What else might you suggest she try to minimize punctures?