I can't offer my experience - I've never fed Iams or Purina. I stay away from foods, treats, etc. made with by-products. But, as someone mentioned earlier, given only these two options, I would go with Purina because of Iams' history of animal testing and cruelty.
There are many quality kibbles out there that are readily available at big box pet stores. In the past, Levi has eaten Blue Buffalo, which you can purchase at Petsmart. No corn, no by-products, and he did very well on the Blue Buffalo. Some Petsmarts have begun to carry Wellness, another quality brand.
We are going the grain-free route now, and I've switched Levi to Orijen, which we are very pleased with.
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