Should I get an insulated steel bottle or a lightweight plastic one for hiking?

It depends on your priorities. Insulated steel bottles like Hydro Flask and Iron Flask keep water cold for hours but add weight, which matters on long ascents. Classic plastic bottles like Nalgene are far lighter and cheaper, and many hikers value the wide mouth for adding ice or scooping water. If pack weight is your main concern, go plastic; if cold water on a hot trail is the priority, go insulated.