Is stainless steel or plastic better for water bottles?

Stainless steel is better for durability, insulation, and avoiding chemical leaching. It keeps drinks cold or hot for hours and lasts years. Plastic (especially BPA free Tritan) is better when you need something lightweight, shatterproof, and affordable. For daily gym use where weight matters, plastic is fine. For all day insulation and long term value, stainless steel wins.