They arent close in popularity. That said you picked them because of release dates. Im picking up both as i want to try Ni No Kuni 2
Ni No Kuni in meta and kirby in sales. Kirby doesnt score that well honestly
They are close. Sure, the Kirby franchise is a lot bigger as a whole, but the last few Kirby games have sold roughly the same as Ni No Kuni 1.