since you have them both would like to see a head to head from your perspective if you have the time.
Okay, it's been a few days. My first impressions are that they are both obviously very fast. But the M8 reacts immediately to my touches where the S5 has a very slight hesitation to it. So slight I don't want to use the dreaded "lag" word. I'm guessing it's still a TouchWiz thing and when I use the S5 a couple of days in a row I don't even notice it anymore. But, when I switch back to the G2 or the M8 I notice the quicker response right away. The S5 is fine and if that's what you use everyday you probably won't care.
The displays are awesome on the S5, the G2, and the M8. They're bright, clear, and have great colors. I really can't say one is better than the others unless I say the M8 and G2 are probably a little brighter in sunlight. The part that does matter to me is the size. As you know, the S5 is designed to give the biggest display area, where the M8 is not. But hey, you have to pay a price for those front speakers on the M8. In the M8's favor though, some apps push the navigation buttons off the screen so the whole display is used. The G2 has some mods that let you shrink the navigation buttons down to a quarter of their original size and even smaller so I have no complaints with the G2.
Those speakers... I use Bluetooth headsets everyday and almost all day at work so I have never cared about speakers. The M8 is my first phone to have front speakers and I have started using them once in a while like when I'm in the shower. The phone sounds great to me and has good volume to it. I'm not sure if it's worth giving up display area to get them. I look at the screen a helluva lot more than I listen to those speakers. It really just depends on personal taste I guess.
Radios... the M8 and S5 have great radios and GPS. My G2 falls far behind. But I hear that other G2's are fine. Maybe I just have a dud. Not much to say here other than that.
Cameras. The S5 has the better camera overall but the M8 is pretty cool if you ask me. I always reduce picture size on phones anyway so the 4mp size is not an issue. I like them both. The G2 is great outside and in bright light but I have trouble getting good indoor shots right. It may be me and not the camera but I don't have trouble with the other two.
The battery on all of them are more than enough to make me happy. I unplug it in the morning and plug it back in when I go to sleep. I don't have to swap batteries anymore but it's good to know I can.
The G2 doesn't have a SD card which is a bit of a let down. But I have to say that if my G2 had a better set of radios it would be a match to the M8 or S5 and might get the lion's share of carry time.
I don't like that the S5 got bigger than the S4 and S3. I don't care about water or dust. I don't need Knox or a fingerprint scanner. I do like the S-Health stuff.
Right now I have the M8 with me, it's S-Off and has Venom running through it. It's hard to put that away for the oversized S5.
http://www.phonearena.com/phones/compare/Samsung-Galaxy-S5,HTC-One-M8,LG-G2/phones/8202,8242,7969