Male cat that recently had crystals in his urine?

Our male neutered cat is about 2 1/2 years old. (We adopted him). About 6 weeks ago, he became ill and spent the day at the vet being"flushed out" because of crystals in his urine. He was put on 2 meds. and given a prescription food, Science Diet, SD. The vet said to keep him on this food for 2 months. OK, I went to pick up another bag of the food yesterday, now the vet said that he will now need Science Diet CD for the rest of his life for maintainence for this problem. I am really new to cats and want to know your experiences with this problem. A friend told me to put him on Purina One Urinary Trac Health Food. What do you all think since I am new to all this? I want to do the best I can for the cat, but is this really necessary or it the vet just trying to push Science Diet as I know they do with dogs. ( I am not a fan of Science Diet to begin with). Any suggestions would be appreciated. Thanks in advance.