There is a good reason why the CW kit is a little more $$$...quality, complete kit (no missing parts, nuts, bolts etc.), excellent assembly manual, lots of options if you want polished billet or stainless parts, fit & finish is the best I've seen, also allows for wider front wheels, Z-bar mount exactly in stock location along with stock engine offset, header quality is beautiful, does not utilize stock Mustang parts, custom steering rack, wheel & suspension geometry re-engineered correctly, inner fender panels fit correctly, stainless button head allen hardware, other than that it's the same. If I was doing another one, I wouldn't have to think twice about which kit to use.
I'm not saying the other kits aren't any good or don't do the job as lots of people use them...I just don't like dealing with missing parts, jury rigging things, and having to deal with "massaging" things to fit correctly. I also like the option of polished stainless and billet aluminum parts.