I have them both at the moment, trying to decide on one. Honestly I've been a bit disappointed in the S5's camera. It's super fast, and dark shots show up lighter (though terribly noisy w/o stabilization mode, and even then and still not as good as the M8 or 5S). But bright outdoor shots are noisier than the S4 when you zoom in a bit. When I compare to outdoor shots on the M8, you sure can't tell a 12 megapixel difference. Indoor shots are difficult to get a focus, even in decent light, unless you have stabilization mode enabled - which forces you to hold the phone still for 3-4 seconds every time. It's like Night Mode from the S4, only you need it more often. The other problem I'm having is load time - it takes me a good 5-6 seconds to go from screen off to ready-to-shoot every time. On the M8 and 5S I can have a shot in 2-3 seconds. It feels like it's hanging, but I can't figure out why.
Other than that, I love the phone - everything about it. The look, the feel, the form factor. I also love the more narrow and rounded feel of the M8, but it is incredibly slippery, and pretty tall. The S5 is not as snappy and responsive as the M8 - but that doesn't mean it's not snappy and responsive. It's an improvement over the S4, the same way the M8 is an improvement over the M7.
Battery life on both are solid, easily getting a day for my usage. The M8 seems to "idle" better than the S5, though.
I've become very hooked on the lock screen gestures on the M8. Once you get used to that type of thing (similar to the Moto X), you get pretty spoiled. It doesn't help that there appears to be a bit of lag with the home button when unlocking the S5 - and yes I have S-Voice disabled.
All of that said, I still like the (get ready for this) more rugged appeal of the S5. With 3 kids and sports and all that, having a dust and water proof phone is pretty nice, so I haven't given up on it yet!
Any help on this lockscreen / camera opening lag would be much appreciated as well, as I'm reading from others that it's not common among everyone. I have the Verizon variant.