I have a lot of experience with struvite crystals because of my cat.
Ok - so here is the basics.
Struvite crystals develop in urine that is too alkaline. Grains (and veggie matter) create alkaline urine. Although C/D is basically corn it uses synthetic dl-methionine to artificially acidify urine. Long-term use of synthetic methionine has been linked to issues absorbing calcium, metabolic disorders and calcium oxalte crystals (which are much harder than struvite to deal with).
Natural methionine is found in meat. Meat based foods often naturally create a more acidic urine, and therefore prevent struvite crystals from forming.
The best diets for animals with struvite issues are raw diets, since they naturally keep urine acidic and provide a high amount of water to keep urine dilute.
I personally wouldn't use Hill's products. C/d has horrible ingredients and you can do much better. My first suggestion is raw, but if your more interested in a kibble i'd look at Orijen or Horizon Legacy - or really any grain-free high meat kibble. As well, you can purchase supplemental Methionine from most health food stores or you can get Wysong Biotic pH-. Don't let your vet push you into Hill's (or Medi-Cal or Purina, etc...) you have a ton of much better and healthier options.
Hope that helps.