I had an iPhone 5s since its launch in September, and recently just got a Galaxy s5 last months. Heres my list of pros and cons.
iPhone 5s Pros:
-Apple EcoSystem
-iMessage
-Everything just feels snappy and I never experienced delays in the software.
-The build quality of the device is second to none.
iPhone 5s Cons:
-Battery life was horrendous on it during usage. Standby time was god-like, but when I used it to browse the internet, the battery drained ridiculous amounts.
-screen was a little too small for my liking
S5 Pros:
-The battery life is great. I normally get about 5.5-6 hours of on screen time and don't have to worry about the device dying.
-The water-resistant design is nice. I actually dropped mine in a pool the second day I had it and no problems.
-The wireless charging cover (extra $30) is awesome. I just throw my phone down onto the charger and it starts charging. No worry about any cables.
-Screen is perfect for me. I have large hands, and I can still use the phone one handed.
S5 Cons:
-Software is sometimes slow, but not anything that will break my experience with the device, yet it is noticeable at times.
-Some people don't like the plastic design (I don't have a problem with it at all).
Either one you choose (or even the m8), all are great phones.