I sprayed 4 goblins silver using Army Painter Plate Mail, and then gave each a quick base coat in my Moria scheme.
I then applied the washes over the whole model to see the effect on normal paints and metallics.
Here are the results:
Dark Tone (Army Painter): Looks the same as BB in liquid form. Is slightly softer in effect than BB, so where details on the model weren't great it looked a bit indistinct. The effect on the metal was very similar to BB (which I love). The overall finish was slightly glossy. The bottle is a dropper bottle which I really like for normal paints but actually prefer a normal pot for washes. This may end up in my old BB pot when it is empty.
Black Shade (Vallejo): Gave a much more matt finish than any of the others and pooled well in recesses giving very clear shadows. It was far more effective on the non-metallic paints - it made the metals look slightly dusty. This wash is also available in a huge pot for only three times the cost of the any of the other washes. This obviously makes it very cost effective, and if you want you could dip the whole model in the pot. (You can buy it as a dropper bottle too like the normal Vallejo range.)
Nuln Oil (Games Workshop): Initially made the whole model look a lot darker than the others and also took much longer to dry than the other washes. It gives a slightly glossy finish and doesn't give quite such clear contours as the Vallejo, but is clearer than the Army Painter. It gives a decent finish on the metallic areas, although not as nice as BB or the Army Painter wash, and the normal paint areas look better than the Army Painter, but not as clear shadows as the Vallejo.
I actually think that at least two of these will find a regular use in my painting. The Army Painter wash will be used on metals, whereas the Vallejo will be used on other paints. The Nuln Oil does a decent enough job as an all rounder and will suit some people who only want one black wash, or who only want to shop at GW. If I had to choose just one I would probably go for the Vallejo wash as the price would just tip it over the edge for me - such great value!