I have a 16 gb iPhone 4s. I am constantly hitting my memory limits, and I fear that the phone will run sluggishly if I install iOS 8. After following today's Apple event, I am thinking of just getting a 32 gb 5s for $149. As much as I'd love a 6, knowing that 16 gb is not enough I'd have to get a 64 gb model, which costs $299. (And that's with a renewed 2-year contract!)
Of course, I could choose to do nothing, but I really want iOS 8. I am, after all, a major iPhone geek, and I want Extensions and Handoff.
I'm much more interested in the latest software than the latest hardware, but I feel I have to invest in more recent hardware to get the best out of the latest iOS update, and I think the 5s will be fine for my needs.