I recommend playing with them in store. Ever since I upgraded to 5.1.1 the UI performance has greatly improved. I don't remember much of my M8, but I'd say it was just as good as the S6 Edge now. If I'm not mistaken, only T-Mobile has 5.1.1 right now, so that's something to consider as well.
I don't have many things connected via Bluetooth, but it has no issues with my Jawbone UP3, VW CC, and my soundbar in my basement. I have also haven't experienced any issues with my back and my phone's gotten warm like crazy.
You can list pros and cons for each, but people will have bias' and these bias' may not be in line with what you care about, so I wouldn't listen to any "fanboyish" post and just listen to owners explaining why they like their phone when not compared to others. For example everything listed above can apply to the iPhone as well. It doesn't really have much value.
My genuine recommendation is to ask users what they dislike about their phones and compare that.