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. Cast aluminum is solid pieces of aluminum welded together. Molten aluminum is poured into molds so the styles are beautifully decorative. With minimal maintenance there's no reason your grandchildren won't be using your cast aluminum set fifty years from now. Because the molten aluminum is poured into carved molds, the detail is incredible.