I’ve been a successful personal strength trainer for many years and would like a certification.
I feel it would come down to how fast you can go over material that is given to you by each certification. Most certifications will require a book as their source and other training manuals that will help. If you are not so good at self study, you may want to look for a reputable certification that has classes. Cooper and a few others do the classes. You can get it done in a couple of months if you apply yourself pretty well.