Can't say anything about the Nexus 6, but I will say the G3 and M8 were the two I debated on for a bit. I went with the M8 and don't regret it a bit. I don't think going to QHD resolution on small screens at this point is worth the expense. Then consider the processor isn't much faster on the G3 compared to the M8 and they have the same graphics chip, I just felt the M8 would be less prone to lag. Some gaming tests backed that up with frame rates dropping on the G3, while remaining full 60fps on the M8.
Plus I'll take front facing stereo speakers over a single rear speaker any day. I know there were other things too, but don't remember off the top of my head. The only thing potentially lacking on the M8 is the camera resolution, but that depends on usage. For me it's fine, and I like the duo effects as well.