Yeah, this is a tiffin vs bento smackdown, I think.
I’ve been using tiffins for years. We are not a microwave-using family. Plus, plastics are well documented bad actors re human endocrine system health. NB: if you live in the U.S., only some bad actor plastics and plasticizers have been banned. If you live in Europe, your governments care more your health and these kinds of things.
So, stainless steel, yes please.
