They both have their strengths. For me, the camera slow down was a huge issue because I just got a new kitten, and he's fast. I was missing more shots than I was getting with my 5S.
Another issue (for me) is how screwed up iCloud is. It hadn't synced a photo to my PC in months without me knowing. I worked with Apple for weeks on it and they weren't able to resolve it. The last straw for me was how unreliable iTunes Match/In the Cloud became. It literally took me a week to put my music back on my phone. I realize that those aren't phone specific issues, but they were issues I would never escape unless I nuked my account or something. With my M8, my photos are being uploaded to Flickr and Google in the background, only on wifi. Just like how iCloud was supposed to work.
There are some apps that I miss, though I do have my iPad mini for those. At the same time, I like tweaking. Just today, I learned how my processor was binned in order to undervolt it; thereby increasing my battery life without taking a performance hit.
I bought a 64gb microSD card, and filed that solely with music. I couldn't do more than maybe 20gb on my 64gb 5S, because I had 10gb of photo apps alone, lol. So I'm getting to hear music I haven't heard in years.
I honestly can say that I don't think you could go wrong with either phone, and it'll be even closer once the 6 is released. . If you want the best accessory market out there and the best of the best apps, and you want a phone that (generally) just works, stay with your 5S. If you like really tweaking and really customizing (sometimes with poor results), give the M8 a good look. You don't have to tweak anything, it works fine as is, but I like getting into the nuts and bolts.
Watch MKBHDs reviews of it, he does a great job.
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