The S5 white balance is a tad cooler than the M8. Did the tester say what the brightness setting was? Auto or Maximum?
Both seem to be excellent displays with the Samsung a tad bluer and the M8 a tad less accurate in terms of chromaticity.
The M8 is known to come with different white balance calibrations depending on the factory and so far there is a neutral one and a cooler one. Since the S5 is supposed to be only a bit too cool, it seems like the M8 model has the warmer version calibration. The version tested on Anandtech I think is less neutral.
I can't wait to do my own testing. The display mate test is not very precise as it gives color temperature but not the color composition at each point of the greyscale. But the average Delta E of the greyscale is very good, which confirms my imression that the green-blue push is much less than on the S4. In the store I tried viewing at different angles and the whites remained pretty clean. Still not as neutral as LCD but much improved. It is the first AMOLED screen I see on a smartphone that could pass as an LCD.
I find it very interesting that as soon as the diplay mate article came out, Samsung-supporting students as well as a couple of web pages immediately published articles quoting the Displaymate paper and spreading the same incorrect rumors about the screen brightness of 698cd/m².
I understand that after the bad publicity because of the dim screen last year, they are anxious to Show they have improved but again using foul tactics...
Look at this: