I don't know too much about this stuff, just what I've been told. From what I understand, all brand cat food that you would have heard of from mainstream advertsing is not very good because it has a high grain or carbohydrate content. While you pretty much have to have some grain content to help bind all the ingredients together in dry kibble, meat should be the primary ingredient(s) for cat food. In nature, cats eat very little, if any, non-meat food. Companies use grains and meat by-products (e.g., hooves, beaks, etc.) because it's much cheaper than using the good parts (like muscle).
As for the brands vets sell, I've been told that vets are animal doctors, not animal nutritionists. They are taught very little about animal nutrition in school. Furthermore, just like drug companies go to doctors and give them free samples to boost their sells, prescription pet food companies go around to vets and give samples and such. Also, I've heard vets make some money from selling the food from their offices. Unless your cat needs a specific prescription diet, you really shouldn't be using a prescription cat food (on the other hand, I've also heard stories of some health conditions clearing up when the cat is switched to a primarily meat-based food). Generally, the brands that vets sell are not considered high-quality (I'm thinking of Hill's Science Diet stuff, which is probably mid-grade).
As for low-vs.mid-vs.high-grade food... the bottom line is that meat costs more than grain. And muscle meat costs more than meat by-products. So, a food that has some form(s) of meat as its primary ingredient(s) will cost more than one that uses some form of grain as its primary ingredient. Also, you want the type of meat (e.g., turkey, chicken, etc.) listed in the ingredients instead of just "meat;" "meat" can mean anything, including dogs and cats that were euthenized or animals that were ill (or so I've heard). For cats, someone I trust who understands all this better than I do recommended any Innova variety. Nature's Variety, Evanger's, and Merrick are also very good. The "lower" quality cat foods also tend to have more preservatives and such in them that may not be good for cats.
All this goes for dogs, too, except dogs are omnivores and can get away with eating more veggies and grains. However, in the wild, dogs still primarily eat meat (think about wolves).
Here's a link to a vet who's been on Oprah for his recommendations with the explanations:
http://www.drmarty.com/feeding.htm. I'm just guessing here, but when you say you looked at the high-quality foods and couldn't see a difference from the grocery store brands, I'd guess that you were looking at things like Eukenueba and Hill's. These are not the highest quality foods; rather, the one's I mentioned above, plus some others, are the highest. However, these brands are not well known and can be difficult to find in stores (next to impossible outside cities). As buying cat food online can be rather costly, I would buy the highest quality you can afford or get and supplement with cooked meat (raw is better, but there's always a risk of disease). Or, if you live in the country like my parents, you can let the cats "naturally supplement" their diets with wild caught birds, mice, squirrels, etc. (sort of kidding here... may be a healthier diet, but the kitties are at the risk of being prey themselves and of catching who knows what from the critters they eat).
Hope this answers your questions!