I personally think that they are on the same level. Not fantastic, but not awful. Both mediocre in my opinion. But Kendall beat Nia when they competed.
Kendall, is a more experienced technical dancer. She dance with high skill. Nia dances as an artist. It depends wheater your looking for an artistic dancer or a technical dancer.
You really shouldn't be arguing about who is better. That really doesn't matter. What only matters is that they have fun dancing. They are just little girls. Don't try to start an argument. :)