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.
Yes. They can also be dangerous in an old pewter flask as pewter was once made with lead. Steel and glass are the best materials for a flask.
No, an oven being stainless steel does not make it work better than a non stainless steel version.
Stainless steel
stainless steel
Potable items that cannot be stored in stainless steel: End of list.
The better conductor is stainless steel.
stainless is better.
Stainless steel is stronger and retards rust unlike iron
Hot drinks are usually put in a thermos flask, so the flask needs to be made of something strong in order not to get ruined in any way, therefor steel is used.
Vacuum insulated stainless steel bottles like one from Hydro Flask [see related link "hydro flask" below] or Thermos [see related link "thermos" below] will keep them coldest.
"Stainless" means that it doesn't rust.
You need to refine your question. Iron and stainless steel both have uses, and so one cannot be deemed "better."