http://en.wikipedia.org/wiki/Apple_Ax
Looking at history, Apple updates the SOC in the iPhone every year like clockwork. Last year, for example, the old 45nm, Cortex A9 based cores in the 4S were upgraded to customized A15s manufactured on a 32nm process in the Iphone 5. Naturally, the next iphone should have a 22nm Cortex A50 dual/quad core CPU with a bigger GPU.
But there's a problem.
TSMC/Global Foundries/Samsung (the people who manufacture iPhone chips) don't have 22nm process tech yet... heck, they're just now getting 28nm right. Unless Apple somehow gets Intel to manufacture it's processors, the 5S simply can't get a major hardware overhaul in time for a summer launch.
As Apple's designing it's own custom chips now, it's hard to predict what we'll even see. At most, we could get a minor update of the current CPU with integrated LTE/ a slightly bigger GPU on 28nm, or it could be exactly the same as the iPhone 5. I'm not sure about other features... fingerprint locks, maybe?
TL;DR: The 5S can't get a big hardware upgrade. It won't be much faster than the Iphone 5. As for useful features, that's completely up in the air, but I haven't heard about anything earth-shattering.
I would wait for the 5S. But buying a 5 now isn't a terrible idea.
