There are different levels of Purely, IIRC from about 40% to 70% meat. I would opt for the food with the highest declared meat content: most cat foods have a high protein content but this can come from an animal source or cheap fillers such as waste products of the cereal industry. Unscrupulous companies will list a few different grains so that they come further down the ingredients list and meat can come first.
At the end of the day cats are obligate carnivores with no nutritional need for carbohydrates; furthermore protein from animal sources is more bioavailable (better absorbed and utilised).
Sorry Sam but I don't trust Royal Canin as they are one of the only 'premium' brands which don't declare the meat content, and won't disclose if you e-mail either.
Where money and extreme fussiness is not a concern, my preference is for Orijen, Applaws, Encore, Porta21 (sensible variant) and similar grain-free foods.