Hi Deborah and welcome to GoY. If this is a living screen you are asking about I'd go for willow.

I agree - willow is more robust (even if not growing).

Everything grows Deborah. Think carefully before planting either of these. A screen with no roots may be a better choice.

Both can grow very vigorously, willow up and out, and bamboo sideways. I agree with Diane, be very sure that's what you want.

Another point, Willow grows vigorously, the roots are the devil of a job to dig out when you want an easy maintenance screen.