I have no experience with the Amarican Autowire kit for Mustangs, but after reading the description on their website, it seems like a very well equipped kit. I have installed one of their universal kits in a 62' Chevy II and I liked it.
The Painless kit has a fuse box that I like better only because it is compact. But the labor to install the kit was not much easier then if I had just installed a universal kit which costs a lot less. So, to me it is not worth the expense.
This guy makes a real nice universal kit. It is high quality and it is inexpensive, if you don't mind GM color codes (most all universal kits are GM color coded)
ZZ-20 Wire Harness
This is not like wiring up a stereo. It is very labor intensive and technical. You must plan and design the harness to fit and rout it neatly. Universal kits typically supply the power circuits for all the switches, but not the harness from the switch to the component. IE: it gives you power to the wiper switch, but you have to build the harness from the switch to the motor. They also have some circuits you might not use, like for power windows or other accessories. Like 2+2 said, unless you're building some Chip Foose custom the original replacement harness is much easier to install. It is also less technical. I have made some good money repairing other peoples wiring nightmares because they tried to get by on the cheap only to get in over their heads installing a kit that was "easy". It's real easy to end up with a bowl of different colored spaghetti, or much worse, an electrical fire.
Bottom line, unless you are a skilled Automotive electrician and do the work yourself, there is no cheap way to re wire a car. If you buy the inexpensive universal kit and pay someone to install it will probably cost more than if you had bought the higher cost factory replacement harness and installed it yourself