I own an iPhone 6 and my girl has the LG G3. HD vs Quad HD display. I get around 6.5 to 7.5 hours of screen time on my 6, and my girlfriend gets 3 to 3.5 hours. Colleague who has the Galaxy S6 with Quad HD says he gets around 4 hours of screen time.
The 6 Plus has 1080p, but is a bigger phone to accommodate a bigger battery to compensate for that higher resolution. The 6/6S would have to be a lot thicker if it had 1080p, because it would need a bigger battery. Also the iPhone 6 has a relatively small 4.7 inch screen, it's just to watch some videos without paying too much attention to the resolution. For reading text, it's perfectly fine. At least, that's how I think about it.