The higher the load, the harder it will be to drive. It also affects the midrange response of the headphone, which is why many pro headhones are 400 ohm. You will not have enough power to drive one with a walkman, but a headphone amp or the headphone amp on a mixing desk will drive them.
So, it depends on what you are using to drive em.
I would play it safe and use 16 ohm, unless you are using an outboard heaphone amp. Hope this helps.