HELP! Which won’t sag?

[bassue]

I’m trying to put up curtains on our patio to diffuse the hot sun. I have a support bracket at both ends and one in the middle that will fit a 1 inch round. From wall to wall the distance is 20 feet but this span is broken up by the middle support bracket so it actually 10 foot spans that I’m struggling with. I have three criteria: it’s outside so it can’t rust when in gets wet, and I absolutely do not want it to sag, and it has to be 1 inch round since that’s what my supports will accept. Can someone tell me if a 1 inch round solid aluminum bar, or a 1 inch round solid stainless steel bar, or a 1 inch round 1/8 inch thick walled stainless steel tube would be better? The supports I’ve put in can hold whatever the weight will be. Can someone help me with this?