In short I prefer the Nexus 6 because:
1. OLED screen. This is a must for me. Also the LCD on the G4 is quite off interms of colour and white balance. Thats on top of LCD's usual gray blacks and lower contrast depth.
2. Stock Android - Lollipop in anything other than stock form just pales in comparison.
3. FF stereo speakers, they really are excellent on the N6 and I watch a lot of YouTube.
4. I always liked Moto and the design of the Moto X. The funky contoured body and the great build quality.
5. Little things like the all black logo-less front and Ambient display. And I really find the larger size suits my use.
The few things in favor of the G4 are its superior camera, sd slot and removable battery. The Nexus 6 is so good that I could look past those features. HTH.