I've been thinking about whether this is just me resisting American English again, after I thought I'd reached acceptance, but I still think "no standing" is better, because, like "no smoking," "no spitting" or "give up this seat to an elderly or disabled person," it directly addresses the passenger with a command, while "no standees" is presumably short for "there should be no standees riding on this bus," a normative comment about the bus, and therefore rather indirect. Also, standees is less likely to be ina foreigner's phrasebook.