If I could chose one feature in which Xiaomi YI absolutely beats SJ4000, its the photography. Images with Xiaomi YI are fantastic. They are made in true 16 megapixels. Colors look natural.This is where Sony sensor comes to the fore.
Even though SJ4000 advertises 12 megapixels photo, in the reality the sensor that camera has can only produce photos in 3.5 megapixels. Images taken with SJ4000 look bleached out and dull when it comes to color. Also they will quickly blur when you zoom them, as they are only 3.5 megapixels.
As far as I concerned, when it comes to still images, Xiaomi Yi is an absolute winner.
But do not just take my word for it. Take a look at photo samples taken side by side and judge yourself.
Obviously, when it comes to blue color, or the sky color, SJ4000 has brighter, and in my personal taste better way to display it. However, when comparing green and most of other colors, YI sensor produces more colorful and natural looking images.
However, when you switch SJCAM SJ4000 Auto White Balance to cloudy, there is not much difference in colors. And I will be honest, I expected YI to preform far better ant totally outperform SJ4000. But once I set white balance manually, here are the results. Not bad at all, right?
And that is the biggest problem. Plenty of online reviewers, when comparing cameras side by side, simply turn on camera take few videos or photo samples, but they do not give a damn to try to learn how to use the product, how to get most out of it. So, as you see in the example above, with simple white balance change, I managed to get nearly as good image from 3.5 megapixels sensor and make it a decent opponent to 16 Megapixel one.
But again, do not take just my word. Here is a link where you can download these exact photos, original files to compare on your own PC. Zoom them out, play with them and if a photo is important feature for you, decide which one is better. Once you download images, those with name starting YDXJ2119 are for Xiaomi YI, and files starting with 2015_0613 for SJ4000.
Just one more thing, SJ4000 produces slightly sharper photos in a long distance. Which mean that objects far a way look slightly sharper. Yi however has far more megapixels, so image will look okay, even when you zoom them in.