Is pewter or stainless steel better for a flask?

Pewter is an alloy made mainly of tin. It used to contain lead, so an antique pewter flask may not be a good idea to use. New Pewter does not contain lead. Pewter can also react with anything that is acidic giving the beverage a metallic taste. It also oxidizes giving it a tarnished look. Stainless steel does not oxidize, and thus will not react with acids giving a visual change in the metal, nor an off-flavor. It is also lighter.

I suggest Stainless steel for a flask. In days of old stainless was not available so pewter was used. Stainless is cheaper and will offer better flavors with acidic beverages.