Both are nickel plated on the WSM.
Stainless will last much longer and to me they clean up much easier than nickel plated.
But I think right now the only grates you can get in stainless are the 22" with hinged sides and the 26" OTG comes with stainless as well. At $125 for the 26", this grate better last me a long time before I replace it.
For a 22" grill the stainless is worth it to me. But for a WSM, the nickel plated is just fine.