Speaking about analogies... How about this car analogy. All cheap cars are made of metal. The most expensive cars (like Lamborghini) have metal frame but use fiberglass for exterior panels. This is how SGSIII is made too. It has metal inside and plastic cover. This is also not dissimilar to real trucks/SUVs (frame construction) vs crossovers (unibody). Although to say that iPhone 5 has unibody is a real stretch since all of its sides are made of distinctly different materials.
Anyhow, your statements about "cheap" Samsung design would have some validity if you could find a single SGSIII owner who experienced problems due to plastic case. Ironically, the phone that cause most problems to its owners because of the case construction is iPhone 4S. Using glass body was a really stupid idea.
I dont mind the s3, its alot better than most androids but all of the show models ive picked up there are already cracks on the back plastic. This is why ill stick to apple for now, because i believe the build is superior