I definitely didn't mean to stir up a big debate, sorry about that!
I was just stating my opinion from the knowledge I've gained working in the vet field and the research I've done (as I want to be a vet).
If anyone is interested, here is what I've learned (but I don't want to stir up any arguments so remember this is just ONE of the many opinions out there!!)
This is what I know from talking to Vets/Nutrition Specialists/Pet food Reps:
1) Science diet and Hill's Prescription Diet are essentially the same foods, the only difference is Science diet is made as a prevenative life-stage product, whereas Hill's Prescription Diet is formulated to treat medical problems. In fact Hill's kitten food and science kitten food contain the exact same ingredients. (The only reason I feed Science Diet and not Hill's is because I don't have my discount anymore at the clinic where I work and Prescription diet food is crazy expensive when you don't need it for a medical issue)
2) Hill's Prescription Diet and Medi-cal/Royal Canin Veterinary lines are really the only 2 lines of veterinary food on the market (Iams makes one too, but it's not as well tested). They both contain "corn" as one of the top ingredients. And while Medical doesn't have the "chicken by-products" that Hill's does, Vets often see better results with Hill's food than with Medical food. While the ingredients may not sound appealing or "high quality" the fact is, these foods are PROVEN to extend lives, prevent health problems and successfully treat a lot of issues.
3) We are starting to see that the new lines of pet food containing "organic high quality ingredients" can actually CAUSE liver and kidney failure because their levels of protein are of a TOO high quality and/or quantity. Also any brand other than Hill's or Medical has the potential to cause urinary stones / crystals. At the clinic I work at, when we ask someone who's pet has this condition what food they are one it is very rarely Hill's, their pet store brand Science Diet, Medical, or their petstore brands Royal Canin and Technical.
I am simply going by experience. I have seen countless cats and dogs come in with problems and I can almost always pinpoint the food as a major factor in it. I know that as pet owners we think that we should go by what sounds healthy/natural/organic but the fact is, the evidence seems to show otherwise.
This is just what I've SEEN. Obviously I'm not a nutriton specialist. Nutrition is a changing field and by the time I am a certified Veteriarian things could completely have changed. So I definitely don't want to offend anyone! Just thought I'd share this info