Both have their own pros and cons. While File Editing will give you a wider range of editable content to choose from, it will require some work to find the right file that needs editing or sometimes even recoding. On the other hand, Load Screen Editing will give you exactly what you want without having to go through the hassle of looking for the file that needs editing. However it has its own limitations as well - some units are not editable anymore because Square Enix has already changed their appearances.