If 1080p in a phone sounds like a lot, that's because it is. When the first iPhone with a "retina display" came out, it packed a 3.5" screen with a resolution of 960x640, giving it 326
pixelsper inch (or ppi). Notably, when Apple increased the physical size of the iPhone to 4", it also increased the resolution to 1136x640 which is still 326ppi. In most cases, 300ppi or so exceeds what the human eye is able to distinguish at a normal viewing distance (which is what Apple's "Retina" is supposed to refer to).
A 1080p phone, on the other hand, has an insane amount of pixels in comparison. In a 5" phone, a display of 1920x1080 has a whopping density of 440ppi. If you go down to, say a 4.7" phone like the HTC One, you reach 468ppi. That's a resolution that's about 50% higher than what most human eyes can readily distinguish under normal usage. You can probably see pixels if you squint really close, but is there any real benefit to going up this high?
High resolution displays use more battery powerIt's impossible to add more pixels to a device without needing power to run them. Some types of panels like AMOLED only light up pixels as they're needed, which means you can conserve some power by using darker themes, but LCD displays will light them all up no matter what, which requires more juice. Not only to physically turn them on, but it also requires battery power to run the processor harder to account for the added complexity (which we'll come back to in a bit).
Just how much extra power does it take? Well, when Apple released the 3rd generation iPad with "retina" (here defined as 264ppi), it came with a whopping 11,666 mAh battery, which was 70% larger than the battery in the previous generation. However, it still only promised the same 10 hours of use. Why? Well, it would be oversimplifying to say it's solely because of the new display, but it's also correct to say that doubling the ppi over the previous tablet (132ppi for the iPad 2) would require a significant increase in power consumption.
You can somewhat see this reflected in Android phones as well. While Android phones vary wildly in battery size, the ones that last the longest are also typically the ones with the biggest batteries. LaptopMag rated the LG G2 as the Android phone with the best battery life around. It also carries a 3,000 mAh battery. Comparatively, the Moto X has a 720p display with 2,200 mAh battery. Both phones will run a full day (and the G2 can actually run longer than that), but larger batteries don't make phones more efficient. They're more akin to Hummers with large tanks than a fuel-efficient hybrid.
High resolution displays use more processing powerWhile powering all those pixels will have a direct effect on battery life, they'll have to go through some processing power on the way. How much is difficult to gauge since there are so many factors that affect performance and efficiency in software. However, as one independent game developer explained to us, increasing the number of pixels in a display will always tax the GPU more:
These higher res screens tax fragment shaders (which do the processing for each pixel) on the GPU more. That means you get worse battery life for the same GPU/battery because for all applications more pixels need to be processed. It also means that in anything that actually gets near the limits of the hardware you have (primarily games), you're going to hit those limits much more quickly. Especially because, in games, the fragment shaders are, the vast majority of the time, going to be the primary bottleneck.
In most cases, we accept this trade-off because things look nicer. However, since the human eye can't tell much of a difference between a 300+ppi display and a 400+ppi display, the extra processing power needed to run those pixels is mostly wasted. Ironically, having a higher resolution display can result in worse-looking graphics purely because the GPU is wasting time on rendering more raw pixels (that you probably can't see), instead of allowing developers to use those resources on adding more elements or details (like the advanced particle systems, lighting effects, and texture mapping you see in advanced AAA games).
High resolution displays are Good for CJK charactersThere are, however, a few benefits to having a high resolution display-like CJK characters. If you haven't heard of CJK characters, then this benefit doesn't apply to you. CJK stands for Chinese, Japanese, or Korean characters. Unlike the Latin-based alphabet that consists of a small collection of a couple dozen or so distinct symbols that create words based on combinations, CJK symbols can be distinct words on their own and potentially bear
striking similarities to different words.
The question of whether or not super high-resolution displays actually aid in reading CJK characters is, as with anything, highly subjective. However, text is one of the first things to become illegible at low-resolutions and CJK characters are considerably more complex than Latin ones.
Keep in mind that, while it's true in an overwhelming majority of cases that a ppi exceeding 300 is overkill, it depends heavily on how good your eyesight is and how far away you view your device. You probably don't normally look at your phone three inches away from your face, but if you do, 300ppi might not be enough.
If you have a need to read text using CJK characters, your best bet is to just look at a phone with a 1080p display yourself, hold it the way you normally hold your phone, and see how it feels for you. However, if you stick with latin-based characters, it's equally likely that trying to distinguish between two screens will just result in your brain playing tricks on you.
Ultimately, your phone choice is up to you and none of them are particularly bad. However, the higher up the ppi in phones get, the more questionable the benefit is. Even if you feel a tangible benefit to a 1080p display in a 5" phone, it won't stop there. LG, for example, recently showed off a 5.5" display with a mind-boggling 538ppi. Arguments about viewing distances, special characters, and "retina" definitions aside, there's an upper limit for phone displays and we're straddling that line now. Chances are for most consumers, worrying about battery life and performance should be a much higher priority
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