After seeing, holding and feeling the M8 vs the LG G3 - which has a similar shape on the back ... I would go with the LG. The M8 back looks great, you know its metal... and the HK-Version with its gold trim looks very nice. The G3, while it has a plastic feel, the texture kind of works for me. I don't really care for the buttons on the back, but ALL LG phones have these now and yet sales are quite good.
What sells me on the G3 is the front. I've seen demo HTC One/M8s with damaged front metal faces. When I rub my finger on the screen, it grating when I make contact with the metal face (top or bottom). LG, Silver or Gold - also has these colors on the top and bottom - but they are UNDER the glass... so you get a single surface.
I still went with the Moto X because of its size, custom color (gone now), the shape and texture of the phone. The features I use on it, works for me... the camera is the weakest link thou... 🙁