I recommend using treated pine deck boards and coated deck screws for these benches for the sturdiness and longevity that these components offer in comparison to the price.
Cedar wood along with stainless steel screws are a great option for a bit of a higher end build.
If you're on a budget, something like regular untreated pine can be used. If you choose this option you will want to be sure to use a good outdoor wood finish to help your furniture withstand the elements.