I don't do stupid things with it.
I'll take the iPhone 6 not the plus because it's way to big.
Is there a lower chance on bending with the iPhone 6 in comparison with the plus?
Yep, there is a lower chance of bending for the 6 because the phone is smaller and has a smaller moment arm for less torque about any pivots.
As for the M8, you said in your list of wants that you wanted a camera but I wouldn't go with the M8 if that was the case. My M7's camera didn't work well at all even in the best of circumstances and I've seen the same with other users at AC. You'll get really soft and blurry pictures because of all that extra light.
The iPhone's camera has freakishly fast focusing so if that is something you want, it is worth considering.
My Galaxy's camera, when I had it, was slow focusing but I'm not sure if they improved things with the S5/Note 4
The 6+ is just as big as the Note 4 so if you are looking for a smaller phone maybe a GS5/iPhone/M8 is best
Good luck with your decision. I would go try out all these bad boys out at the store to get a feel for all of them along with visiting their respective forums to see the kind of issues you might have