Concerning the plates, my inexpert opinion...
To me there still seems to be a significant difference in the aesthetic quality of the plates in the latest NatGeo guide compared with the benchmark Svensson et al (Collins Bird Guide etc). I find it hard to put my finger on the reason - perhaps it's partly a result of different printing techniques?
Svensson et al mostly gives an impression of exquisite fine detail, with subtle, realistic colours - always easy on the eye. In comparison, although the new artwork is definitely an improvement, the NatGeo plates often seem very 'heavy' - bold and dark - as if the contrast is too high.
Sibley is closer to Svensson et al - with a similar lightness of touch and pleasing shades, but perhaps less forensic detail.