I can give an incomplete answer about Science Diet, and wait for someone else to fill in my gaps.
A lot of vets know very little about nutrition - the last vet I saw tried to get me to feed my cats Nine Lives
Here are the first few ingredients in their Chicken and Rice Formula: Chicken, Brewers Rice, Corn Gluten Meal, Chicken By-Product Meal, Ground Whole Grain Corn,
Now, while they live up to their claim that real meat is the first ingredient, the rest of the ingredients are only so-so. Corn tends to be quite indigestible, and if I remember correctly some cats are allergic to it. So a good cat food has no corn in it at all. Also, animal by-products are generally scary things - they are the leftovers after processing is done on the meat. Its usually compared to the sludge scraped off the factory floor and may include feet and beaks and such.
So when looking for a good cat food, you want to maximize the amount of meat present, avoid corn and wheat, and stay away from by-products as much as possible. Hopefully, that helps for those who have a limited selection find the best food possible.