Overall, Royal Canin is better than Pro Plan, but both are low quality foods.
Purina Pro Plan Salmon & Rice Ingredients:
Salmon, brewers rice, corn gluten meal, poultry by-product meal, animal fat preserved with mixed-tocopherols (form of Vitamin E), wheat gluten, egg product, wheat flour, fish meal, soybean meal, brewers dried yeast, calcium carbonate, phosphoric acid, potassium chloride, animal digest, tetra sodium pyrophosphate, salt, choline chloride, Vitamin E supplement, taurine, zinc sulfate, ascorbic acid (source of Vitamin C), ferrous sulfate, manganese sulfate, niacin, Vitamin A supplement, calcium pantothenate, thiamine mononitrate, copper sulfate, riboflavin supplement, Vitamin B-12 supplement, pyridoxine hydrochloride, folic acid, Vitamin D-3 supplement, calcium iodate, biotin, menadione sodium bisulfite complex (source of Vitamin K activity), sodium selenite.
Brewer's rice, corn gluten meal, poultry by-product meal, animal fat, wheat gluten, egg products, soybean meal, brewer's yeast and menadione sodium bisulfite complex are all BAD ingredients! Plus, salmon should NOT be the main source of protein. It contains little nutrients. Poultry, beef, lamb, etc should be the main source of protein, not fish.
Royal Canin Special 33 Ingredients:
Chicken meal, chicken fat (naturally preserved with mixed tocopherols, rosemary extract and citric acid), rice, corn gluten meal, corn, chicken, natural chicken flavor, wheat gluten, beet pulp (sugar removed), dried brewers yeast, salmon oil, dried egg powder, soya oil, pea fiber, fructo-oligosaccharides, potassium chloride, calcium sulfate, sodium silico aluminate, DL-methionine, choline chloride, L-lysine, sodium chloride, sodium tripolyphosphate, taurine, Vitamins [dl-alpha tocopherol (source of vitamin E), L-ascorbyl-2-polyphosphate (source of vitamin C*), niacin, biotin, riboflavin (vitamin B2), d-calcium pantothenate, pyridoxine hydrochloride (vitamin B6), thiamine mononitrate (vitamin B1), vitamin B12 supplement, vitamin A acetate, vitamin D3 supplement, folic acid], Trace Minerals [zinc proteinate, zinc oxide, ferrous sulfate, copper proteinate, copper sulfate, manganese proteinate, manganous oxide, sodium selenite, calcium iodate].
Not a whole lot better, really. Corn gluten meal, corn, wheat gluten, beet pulp, brewer's yeast and soya oil are YUCK! Atleast the two first ingredients are meat. I honestly suggest switching both to a higher quality food. Compare those ingredients to a higher quality food, Innova EVO:
Dried Chicory Root
Viable Naturally Occurring Microorganisms
As you can see, Innova EVO is much healthier than RC and Pro Plan. Actually, I don't care for any commercial pet food, but if I had to feed it, I'd feed Innova EVO. I currently feed a raw meat diet. But if you must choose between RC or Pro Plan, go with RC. Good luck!