There's rumours going around that the Z4 will be closer to the Nexus 6 in size while the M9 will remain closer to 5", so I'll probably go for the HTC. If Samsung release a de-bloated S6 (i.e. no touchwiz, or seriously thinned out of useless apps) I'd give it a try but in any scenario it's going to be hard getting used to an OEM skin again.
I kind of feel the same way. I hate oem skins with a passion. I'm sure there will be a device next year that will give users google android. Just not sure who will be making it. I really want google to come out with 2 different Nexus phones next year, like Apple did with its iphone. Just have to wait and hope.