its a slight improvement on the iphone 5 in terms of the front facing camera and i think processor and apple will no longer sell the 5, if you can afford to push the extra £60 or so, i would get the 5s.
I ummed and ahhed about whether to get c or s. Went with a in the end as it wasn't much more than the c and same monthly cost.
Gotta say, I'm loving it. DH has commented on how much faster than his 5 it is. I love the fingerprint recognition. I work on the idea that technology changes so quickly that (cost dependant) you get the best one you can especially when I'm gonna have it for the next 2 years