Marc Edwards charted out a month’s worth of steps counted by both the M7 in his iPhone and his FitBit Zip. The stats are extremely close.
Methinks if you’re casually interested in step counting and keep your iPhone in your pocket most of the time, it might be worth saving the money you’d spend on a pedometer and put that towards a 5s (if you don’t already have one).
P.S. Pedometer++ is a pretty cool, and free, app for tracking steps on the iPhone 5s.