Powder-coated aluminum is light, rust-proof, and the most common frame material in mid-to-upper-tier outdoor furniture; it holds up well in most climates but can dent. Teak is naturally weather- and rot-resistant without any coating and silvers to a gray patina over time unless oiled regularly — a maintenance choice, not a defect. Wrought or tubular steel is heavier and sturdier in wind but will rust at any scratch or chip in the coating unless it's stainless or properly galvanized. Synthetic wicker (woven high-density polyethylene, or HDPE, over an aluminum frame) resists UV fading far better than natural wicker or rattan, which really only suit a covered porch.
Cushion fabric matters as much as frame material outdoors. Solution-dyed acrylic (Sunbrella and similar) has color woven through the fiber rather than printed on top, so it resists fading in direct sun for years; printed polyester fades noticeably within one or two seasons of full sun exposure. Foam inside outdoor cushions should be quick-dry, open-cell foam that lets water pass through rather than standard indoor foam, which holds water and mildews.
In coastal climates, look specifically for marine-grade stainless steel hardware (grade 316, not 304), since standard stainless will still corrode in salt air. In hot, high-UV climates, darker frame finishes and darker synthetic wicker can get uncomfortably hot to the touch by midafternoon — worth asking about if the set will sit in full sun.
A store that talks through your specific climate and sun exposure before recommending a frame material is doing more useful work than one that just points you to whatever's on the floor.