If you want reliable, mature, error free performance, then the m8 is the way to go.
With the G3, LG is chasing specs to the detriment of the user experience. The g3's engine can't keep up with the screen and performance suffers greatly. I want a smooth, cohesive, reliable user experience. G3 cannot supply that. I don't want to have to wait until my device cools down sufficiently before I can resume watching my videos. Or constantly worry about overheating and watching the screen dim in order to aid in cooling. I don't want a battery that suffers dramatically due to all the juice the uneccessarily juiced up screen requires.
The m8 offers a consistent performance. Day in and day out. The G3 is just too buggy. I'm amazed LG released it in that condition. Chasing specs. User experience be damned.