50f1.8 ?
that is a great little lens. for about three years it was my favorite lens. now I have three L lenses and a fisheye, and while they are excellent, most of my photos that hang on the wall are from that one hundred dollar plastic lens. but part of that might be laziness about printing out new prints and framing them.
And if you believe DXO, it is rated higher than the 600F4 V2...... And at only 1/80th the price.. 
That's why I don't believe DXO ... not saying the 50mm f/1.8 is a bad lens.
I do believe DxO. But you have to uderstand what their numbers mean, and that's the problem - most people don't take the time to do so, and dismiss useful information out of hand.
The 600 II is better in terms of sharpness, distortion, vignetting, and CA. The 50/1.8 has higher transmission...and a higher 'DxOMark Score'. Why? As they state, "
The DxOMark Score is measured for defined exposure conditions corresponding to low-light scene with 150 lux illumination and an exposure time of 1/60s. These conditions were chosen as we believe low-light performances are very important for today’s photography and it is also important for photographers to know how well lenses perform at the widest aperture." At 150 lux and 1/60 s, which will perform better - an f/1.8 lens or an f/4 lens? That's why the 50/1.8 is 'better'. For most of us, the 'Score' isn't often relevant. However, the
Measurements are a useful tool to compare lenses.
