Hi Ronnie,
In term of viscosity, Vallejo Model Colour line is quite thick and must be thinned before airbrushing. The Mig AMMO line is thinner and could be airbrushed out of the bottle. In fact it is more comparable with the Vallejo Model Air line which was like pre-thinned for airbrushing out of the bottle.
Thinning with water is okay but better result with respective thinners from Vallejo & Mig AMMO.
Frankly, they are both very good. It all depends on the range of colours available. I usually use a mix of both sides.
Paul