Welcome Tex - I suggest you start by becoming familiar with the search function (top right corner) on Club Cobra - there is an infinite library of information to answer your basic question.
The ERA and Factory Five cars are different concepts - FF5's basic idea is donor build using an '88-93 Mustang parts car. An ERA is essentially a factory built car that is "faithful to the original in the areas you can see, and updated in the areas you can't see".
There are also a number of other manufacturers that will have your head spinning before you decide which way to go - be sure to look at Unique Motorcars, as one other example.
Factory Five owners also have their own active web site -
http://www.ffcobra.com/.
Good luck, take your time, and have fun!
Bruce
