Whatever material will last the longest and need the least amount of maintenance.

IPE is great and I lkke that you have 2 choices you can let it weather to gray or choose to reoil every so often depending on where you are . All decks get hot in the sune we actually just covered all our cedar decks with fake grass it has drainage every 4 " nad is a bit coolor to walk on around the pool but still gets hot I do love it though . We only did the deck that makes sense that it might be grass . I love not having to redo mt pool deck every year we did cedar . We live where the summers are hot and the deck boards just warped even with the under the deck attachment that are supposed to be better . Friends did IPE last year and they love it and plan to just let it weather.

Most sustainable would be Southern Yellow Pine that has been pressure treated. It is a fast growing tree, but due to that you have to soak it with possibly noxious chemicals. Cedar is lovely but soft and a meal for bugs. Ipe is stunning but a rainforest hardwood. Will last several times longer than even treated syp so is that more sustainable? Or use a synthetic wood that will last forever (longer than the framing) and has 'some' recycled content.

What is available in your area?

IPE and Composite both are great. Composite will give a natural wood-like finish, and also doesn't require high maintenance. IPE is known for its durability and also requires little to no maintenance.