Well, it is my opinion that none of those are better nor premium foods -- Iams IS a grocery store food btw.

These are the ingredients to President's Choice, which is a Supermarket food found at Superstore, Extra Foods, etc. IT can only be bought in grocery stores.
Chicken, chicken meal, brewers rice, ground corn, corn gluten meal, poultry fat (naturally preserved with mixed tocopherols, a source of vitamin E), ground barely, fish meal, poultry flavour, flaxseed meal, yeast culture, dicalcium phosphate, potassium choride, dried whole egg, salt, mannannoligosaccharide, choline chloride, calcium carbonate.
Now, a prescription formula of Science Diet that can only be found at the vet.
Ground Whole Grain Corn, Pork Fat (preserved with mixed tocopherols and citric acid), Chicken By-Product Meal, Soybean Meal, Corn Gluten Meal, Soybean Mill Run, Chicken Liver Flavor, Calcium Sulfate, Dried Egg Product, Soybean Oil, Flaxseed, Iodized Salt, Dicalcium Phosphate, Choline Chloride, Potassium Chloride, Vitamin E Supplement, vitamins (L-Ascorbyl-2-Polyphosphate (source of vitamin C), Vitamin E Supplement, Niacin, Thiamine Mononitrate, Vitamin A Supplement, Calcium Pantothenate, Biotin, Vitamin B12 Supplement, Pyridoxine Hydrochloride, Riboflavin, Folic Acid, Vitamin D3 Supplement), preserved with BHT and BHA, Taurine, minerals (Ferrous Sulfate, Zinc Oxide, Copper Sulfate, Manganous Oxide, Calcium Iodate, Sodium Selenite), Beta-Carotene.
I'd pick the grocery store brand over the Science Diet any day. In fact, I rarely find a grocery store brand that has less natural ingredients than Science Diet...Even Beneful's ingredients are better than that, as well as most of the other Purina formulas. Even Pedigree has some better forumlas than that. None of these are "premium" foods, yet when compared to Science Diet appear much better. Is Science Diet still a "premium" food?