I was surprised to hear that plastic and metal bottle weights are nearly the same. Given that, I'll go metal for the heating/ boiling option.
Who told you that?
I just ran some comps.
My plastic 32 oz Nalgene bottle weighs 6.3 oz w/ lid. (personal test)
This link shows as
38 oz Guyot Design Stainless weighs 12.9 (prob w/ lid)
This link shows
Nalgenes Stainless 1L also weighs 12.9 oz.