It's light, low maintenance, non-rusting and versatile. An aluminum frame can have a cushion, a mesh sling or vinyl straps. Many styles will stack if storage is a consideration for you. A strong factor is also maintenance. An aluminum sling chair is easier to keep up than most other options.
Slings are scrub-able making any mildew easy to remove.
Because the sling dries out right away you get less mildew anyway. Slings are easily replaced. For by the pool, sling chairs are the best. Cushions are fine for outside too but the thicker the cushion the longer it takes to dry out. Most people agree that cushions are more comfortable.