I've heard a rumour that Capaldi has found filming 13 episodes all in one go quite tiring and that he might not return for Series 9, so they've said that they'll split Series 9 in two so that it'll give him a break in between. Again, just a rumour I heard from a friend so I have no idea how true it is but if it is true then I'd rather have a mid-season break in order to keep Capaldi than only have him for one season.