My thought is that you can not compare the two.
The S3 makes decent photo's and what you see is what you get.
With the M8, because of the ultrapixels you get a picture with some sort of a grain, and you have to take more effort to get the picture you want. (maybe it's because i'm more skeptical since i've had the S3)
I personally love the photo's i take with the M8, they give me the feeling that im dealing with an old camera (in a positive way)