Will they have a finish? SYP will rot pretty quickly outdoors unless it has a protective finish or is pressure treated. BTW, si ce price is a consideration, that would be my recommendation PT SYP.

I build mine from mostly ERC as it's abundant and available around here. I've also built them from Cypress and Walnut. The Cypress chair is the only one that has failed from weathering.

I also recommend sealing the leg bottoms with epoxy to prevent water wicking, regardless of the wood used.