I want to know which is the most popular , I cant decide out of the midnight opal and the oyster, which one is nicer??

I can honestly say both colors would look great with the rich dark brown of the walnut cabinet. I would really suggest considering whether you would like the look of a light color (Oyster Opal) or a dark color (Midnight Opal) for when your spa is open. Both are popular colors, but I personally like the Oyster a bit more.