I usually will keep the same counter top if I mix the cabinet colors. If I am keeping one color of cabinets, then I sometimes will do a different CONTRASTING stone finish on the island. Using 2 different white will look off. Keep in mind that your accessories will bring the kitchen to life. It's easy to get excited about all of the finish choices, but in the end, you want the kitchen to be a backdrop to your life.

Do you have any suggestions regarding MSI Stone's Pelican White versus their Marbella White?

I agree with Susan Berry on the mixing of the whites, using more than just one may make it look like they just ran out of one, so you had to pick another. Of the 2 colors you have selected, I like the pattern in the Pelican White. I think it will provide more of a pop against the white cabinets and better accent the gray that you're putting on the island.

I think you have your answer already, but I'm also a no on the white / white mix. Compromise somewhere else and let him pick the backsplash or something.

You can mix counters with all the same cabinets or you can mix cabinet colors and the same counters but I do not ever mix both

Have you considered doing a wooden butcher block top on the island. It can be stained any color. This is one of my kitchen designs and stone hood designs. I like the finish on the wood island.. This kitchen was huge, so we did get away with contrasting counter tops and contrasting counters.