I have absolutely no issue with the phone being too dim. I use auto in and outdoors with no problem.
The text has a sharp effect on grey pages I think. You can notice it but it's not as bad as some people make it seem.
I didn't consider the M8 at all. My debate was between the S5 and G3. The m8 looks nice but aluminum scratches fairly easy, it's quick heavy(I wouldn't want that in my gym shorts), average at best camera. I just wasn't feeling the M8. Its big plus are gesture feats, speakers, and battery life from what I've heard.
I love my G3. No phone can compare to its aesthetics, fantastic camera, double tap to wake and lock works flawlessly, the keyboard is actually really good, the display, the battery easily gets me through a day, I love the animations, lots of customization options out of the box.