the practical difference is if your tension is too low the 9/10 speed will drop where the 1 speed won't.

but even then at low tension there's only a quantum probability of it happening. the realistic difference is do you care and do you have a safety hazard if your chain drops?

but you know this so i can only assume you're starting the thread for discussion.

so i'll muse: isn't having faith in your equipment more important than differences between functional setups? a 9 speed chain will be functional and if it's tensioned there's no way it will come off. so isn't the thought that "oh it's made to stay on the cog" more of the issue?