Should I get an insulated stainless bottle or a plastic Nalgene for hiking?

It's a weight-vs-temperature tradeoff. Insulated stainless (Hydro Flask, Iron Flask) keeps water cold for a full day of hiking but adds significant weight and cost. Plastic Nalgenes are far lighter, cheaper, and accept boiling water for purification or cooking, but offer zero thermal protection. Backpackers carrying a stove tend to favor plastic; day-hikers in hot weather tend to favor insulated.