Probably sticking with the 5S, as I've been on the "S" cycle ever since the 3GS back in 2009. With iOS 8 on the 5S, it does everything I need. As far as a bigger screen, I guess I can't exactly miss what I never had. Same as when I passed on the 5 and held onto my 4S from 2011 until the 5S came out last fall.
Although I am tempted somewhat by AT&T's trade-in promo to get $200 credit for trading my 4S, which is absolutely unheard of, I will likely stay put.
By the time the inevitable 6S (and 6S+, perhaps?) comes out next year, I'll be ready and have an upgrade waiting.