The only answer is both. Nasti is the current selection and deservedly so but we really need quality in reserve and that's Joleon.
They are very different types of player though with Joleon being the old fashioned stopper and Nasti being a bit more accomplished on the ball.I would say that Nasti isn't as good on the ball as some would make out.More comfortable for sure but his passing isn't that great just as Joleon's isn't.His job is to d efend and give the ball to Barry or Yaya and they both do that well enough.
The big difference in Joleon's favour is his strength and aerial ability.In the away games at Newcastle and Norwich we were struggling with the basic lump into the box and as soon as Joleon came on we improved massively.Nasti is ok in the air but so is Kolo yet it's highlighted as a Kolo weakness!