"Spam Can" tends to be used of the mass market American aircraft such as C150 to 172 and PA 28, particularly by arrogant gits such as me who fly things made out of wood. The term is more or less derogatory, but can be used affectionately, as in "G-SPAM is only a spamcan, but we love it". It's not just about being made of metal, as people would not tend to call Socatas, Cirruses (Cirrii?), Pups, Rallyes, Kolibers, and so forth, spam cans.
The heirarchy is as with cars, and goes as follows:-
(1) Whatever you fly yourself is great.
(2) Whatever other people fly, if not the same as (1), is a POS.
Does this help?