You might want to check out this article:Cat Food Reviews
It's short, sweet, to the point...and not biased toward price snobbery. It DOES make the point that you should buy the best food you can afford, and that there are real differences in pet foods. It gives Purina Pro Plan good marks, although they rank it in the "low-to-moderate-priced" foods. This article has two links in it to discuss higher-priced foods, and the different types of foods and the reasoning behind them.
A crucial point is to choose a food that your cat will eat. Cats DO have preferences. Punkin will eat next to nothing, although I DID see him eating out of Ella and Sterling's bowl this morning, instead of his own. This is promising; their food is less fattening than his, and he could use some weight control.