Okay, I have to admit that I have not read all of the reviews/first impressions for these two devices.
However, I can say that I currently have (and use) both the M8 and the S5: M8 is my personal phone, the S5 is my work phone. I do not use my S5 very often, MAYBE 10 minutes of screen-on time per day (mostly checking emails or taking pictures to remember things). A full charge on my S5 gets me through 3-4 days, with 99.99% of that time being screen-off (and also no real app usage aside from email and camera/gallery).
For my M8, I have never gotten below 30% on my battery during a full day of use. I frequently check facebook, texts, gmail, stream music via bluetooth to my car's radio, play games, etc. I unplug the charger at about 7:15AM every day and do not charge again until ~10PM at night. I have wifi on all day (connected at work and at home). Typically, I have ~60% battery left after a full day (~2 hours of streaming music to my car radio and another 2-3 hours screen-on time).
For me, I'm not considering upgrading to either the M9 or the S6. I like both of my current devices. They work perfectly for what I need them for. However, if I had to choose between the two for personal preference, I would not hesitate to pick my M8. I love the Sense interface (minus Blinkfeed). I love the screen size and device size - the S5 really is too big for me. I love the solid aluminum construction, even without a removable battery. The M8 has been perfect for me and aside from a few minor complaints that honestly aren't even worth listing, I can't think of how this phone could be any better. The M9, with a larger battery and more power-efficient CPU should be better than the M8 in battery life, but that's never been an issue for me to begin with.
TL;DR version: S5 has good battery. M8 more than gets me through the day. I prefer the M8 over the S5 and would choose the M9 over the S6.