I agree that IVD is the better quality product. The ingredients aren't great but at least it's owned by Royal Canin, which is basically a good company that I consider trustworthy. I don't consider Hill's trustworthy at all.
The most, most important thing for crystals is a diet of canned food only with dry food as an occasional treat if at all.
The problem with treating struvite crystals with prescription food is that it can cause the urinary pH to swing in the opposite direction which sets you up for calcium oxylate crystals. Oxylate crystals are even worse because they don't dissolve and usually require surgery.
So I would use canned prescription food only until the crystals dissolve, then put both cats on a very high quality canned diet. Not the kind that you can buy at the grocery store - they are all poor quality foods with subpar ingredients. Go to a pet specialty store or human health food store (like Whole Foods) and get the top notch foods. Examples of these foods are Felidae, Eagle Pack, Innova, Avoderm, Chicken Soup, Merrick, Wellness, Trader Joe's, Whole Paws, Pet Guard, Newman's Organic, Evolve, Evangers, Precise, Natural Balance, Nature's Variety, Lick Your Chops, and other foods with similar very high quality ingredients.