Are stainless valves harder on stock iron head seats?

There are many grades of stainless. Some are soft, some are very hard. Most stainless bolts are soft and will ring off easily. I have some stainless plates that are so hard you cant drill or cut it without annealing first. So it really depends on the exact type of stainless they are made of.
As far as your question, I really dont know. As mentioned, its probably pointless to go stainless if your running stock induction hardened seats. At minimum you should put hardened seats on the exhaust side since they take more of a beating.