I recently eschewed the additional expense of the 8, in favour of a cracking deal on a 7s. There's pretty much FA between most of the iphones from what I can discern, aside from louder speakers and camera improvements. From my perspective, if you're that bothered about picture quality from your phone then stop being a knob and go all out on a proper 35mm job.
7S - 10mb, £50 quid down and..... 27 quid a month.. Now, unless you're 14 and use snapchat or Instagram to excess, this is an absolute no brainer for a chap in his 50's.