When all is said and done Splatoon will end up selling more. I think sales for Splatoon 2 will even be bigger. It will establish itself as one of the biggest franchises in Japan for next generation.
Funny in the comment section of the IGN review for FF15 I had a discussion with a guy who really believed FF15 would outsell pokemon sun/moon. That was two days prior to the first week sales. I haven't heard from him since...