I have both the m8 and g3.
If u intend to get g3, be sure to check it out carefully at the shops before buying. Yes, it is better in every possible specs you can find but make sure your hands are comfortable in using the large 5.5" screen. Its with minimum bezels but still handling a large screen is not easy. Also, I find it is a bit too sharp to my liking, tends to tire my eyes easily after using for more than 10 minutes.
If really comparing to m8, I find m8 a better phone to use as a daily driver. Its UI is way smoother (even faster and smoother than stock KK), its camera seems to capture faster and the smaller screen is more manageable and honestly I don't find the QHD screen of g3 has any advantage over the FHD of m8 by normal usage. Of course unless if I look at the screen with my nose touching it, maybe I can see pixels then.
Just simple comparison of opening built in stock apps like phone, message, chrome and gallery side by side, the m8 wins hands down. It has no hesitation at all while g3 seems to need 0.5 or 1s to think before executing the task you throw at it.