If you have experience with the older line of
GW washes, hopefully this will help:
Soft Body Black is weaker than the old
GW Badab Black pigment-wise but more or less comparable replacement. Baby Poop is pretty unique as far as color goes, but fairly strong pigment wise. When I use Baby Poop or Armor Wash (which I love, by the way) I typically thin it 1:1 with water. I have a bunch of SWM washes and I highly recommend them.
The Army Painter washes are pretty damn close replacements to the beloved
GW Devlan Mud and Badab Black. Here is a comparison to the
AP washes to the old
GW washes:
http://fromthewarp.blogspot.com/2012/05/devlan-mud-and-badab-black-wash.html GW Nuln Oil is more heavily pigmented than Badab Black (which it replaced) and I find that I
usually thin it 1:1 with water. I have no experience with Argrax Earthsade, but I do know it was meant as a replacement for Devlan Mud, but the colors don't that closely.
As far as justifying their purchase over Vallejo, I can't answer that one for you. I would say that really depends on how much you use washes and what you use them for. Personally, I use acrylic washes very sparingly so I have no qualms buying a more expensive-by-volume product so
YMMV.