Aegislash is better due to it having good matchups combined with its great typing, Ghost/Steel it has solid attacking stats, it changes form depending on what moves your using so it can be useful in certain situations.
Ninetales is inferior because although its not bad there are other Fire type options to chose from because its only really good in special defense and speed with having only a decent special attack stat for playthroughs, for a Fire type I would suggest Ceintiskorch, Hopefully this helps :)
