The FAT 50 is deeper & taller than a regular 50 and will give you an extra shelf on the inside, so it basically doubles what you can cook on a regular 50.
The 60 series comes with an option for an upper shelf and overall it is a bigger unit, so it will cook even more than a FAT 50.
For competing and just doing smaller events 400 and less, the FAT 50 is the best unit. I personally have cooked for 1000 using a FAT 50 before. I wouldn't recommend that, but it has been done.
Our biggest unit, the 70 is the unit to go with if you will be catering for large groups on a regular basis.
Hope this helps answer some of your questions.
Mike