There is a lot going on in dpreview forums regarding D800 and 5D3's dxomark score. 5Ds's dxomark score is considerably lower than D800 - but to be honest this dxomark score does not tell a lot about their performances, instead of looking at the score, one should look at the graphs of different parameters - DR, SNR, color depths etc.
From what I see, D800 has advantages in these areas:
1. Significantly higher DR at lower ISOs, you will see the difference if you shoot scenes with high DR, otherwise not.
2. Less chroma noise than 5D3. This difference should be visible only at higher ISOs (even though the difference exists at lower ISOs).
Other D800 advantages:
1. $500 cheaper
2. Brightness/ ISO compensation works with Auto ISO on manual mode [extremely useful for me].
3. Can detect faces in phase detection AF [extremely useful feature for me]. 5D3 can't do this. So 5D's auto area AF is much less useful for portraits
4. DR optimizer can work on both highlights and shadows at the same time. In case of Canon, if you turn on the HTP (highlight expansion), the ALO (shadow range optimizer) will not work and vice versa.
5. Can use crop sensor (DX) lenses.
6. Higher resolution - can be used to print larger or crop more at lower ISOs [not important for me].
5D3 advantages:
1. Better well rounded raw converter bundled with camera.
2. Smaller raws can be very useful for event shooters.
3. Better LCD.
From what I see, D800 has advantages in these areas:
1. Significantly higher DR at lower ISOs, you will see the difference if you shoot scenes with high DR, otherwise not.
2. Less chroma noise than 5D3. This difference should be visible only at higher ISOs (even though the difference exists at lower ISOs).
Other D800 advantages:
1. $500 cheaper
2. Brightness/ ISO compensation works with Auto ISO on manual mode [extremely useful for me].
3. Can detect faces in phase detection AF [extremely useful feature for me]. 5D3 can't do this. So 5D's auto area AF is much less useful for portraits
4. DR optimizer can work on both highlights and shadows at the same time. In case of Canon, if you turn on the HTP (highlight expansion), the ALO (shadow range optimizer) will not work and vice versa.
5. Can use crop sensor (DX) lenses.
6. Higher resolution - can be used to print larger or crop more at lower ISOs [not important for me].
5D3 advantages:
1. Better well rounded raw converter bundled with camera.
2. Smaller raws can be very useful for event shooters.
3. Better LCD.