I've heard lots of conflicting info about cat foods too, and what I've been able to glean from them is this... As long as your cat is getting the overall nutrition it needs, I don't think the actual type of food matters. But if your vet says hard food, he's probably thinking of the dental benefits for your cat. The hard food can clean off some of the tartar buildup while canned just adds to it.
I'm currently feeding my two furbabes Iams simply because anything else (and I've tried almost all of them) gives one of them mad gas and awful diarhea, even when the transition is gradual. If you're still really concerned about this, I'd talk to your vet in depth about it. He/She could probably reccommend the best for your cat's needs.