Honestly, it sounds like your vet doesn't know a whole lot on nutrition and dog food. The first red flag was when she recommended Science Diet. Has the vet ever even LOOKED at the ingredients? What kind of medical practitioner would recommend mineral/vitamin pumped corn meal and carcinogenic preservatives for a primarily carnivorous canine? IMO, not one who is well versed in nutrition.

Since I have rather limited experience with UTIs, I can't suggest which food is better than which in your case, but I would recommend trying to stick to the upper level brands. Now, if my memory serves me correct, I believe protein can be a factor in UTIs...Or perhaps it was more phosphorous...Whatever it is, I would hope that Sadie is on antibiotics.

Also, you should check the nutritional guidelines for ProPlan and CA Natural. If they're similar, I would just suggest sticking to a upper level food that contains similar percentages, but perhaps with a different protein source. ProPlan isn't the worst, but it ain't the best, either! In my honest opinion, CA Natural is one of the best foods

Edited: I remember reading about a dog who had stones, but was then switched to Life's Abundance. The dog did rather well on it, but I don't know the ingredients list off the top of my head. Perhaps you can look into that.