I've had my M8 for just over a week. I got to use the S5 at my local Carphone Warehouse for 20 mins yesterday.
Both phones IMO are very good. It really boils down to preference. I think you have to look at core features first.
Build - metal vs functional but gain water resistance and dust resistance. Do you want a phone that looks like a piece of art? Or is utilitarian.
Audio or camera - which is more important. Having great audio on headphones and loudspeaker makes a major improvement on every app that uses sound. Or does high pixel count vs effects come first? This one I can not decide as the M8 photos have been very good , but I know the S5 camera should be better.
Pedometer - it seems the fitbit inside the M8 is always on with no battery loss. No sure the S5 can operate the same way without having to turn on and off etc
IR remote - the HTC M8 has a much more powerful and nicer software for the infra red remote.
Sense vs TW - I like both. Sense is much lighter and therefore makes the M8 faster in use. In real life you won't be comparing. So it won't matter.
Miscellaneous - and then it's all the others extra bloatware of the S5 that the M8 doesn't have. I don't class it myself as bloatware but as options you can decide to use or not.
I see in my eyes the M8 at crafting and finessing a selection of aspects for a phone, whereas the S5 is a master of none but does give you everything.
It's a very tough call to choose.
My last thought was how comfortable and secure the S5 felt in the hand vs the M8. Being able to fit the entire width and height in your hand is a good thing. But then you don't get amplified speakers .....
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