re: Goal line Defense

It really depends on the offense you're facing and your personnel. Really the things to keep in mind:

Pros for Man:
Less personnel to cover receivers
Ability to play press on those receivers
Ability to adjust your defense to cover a player instead of focusing on a particular area
Better suited if your Corners and LB's are really good

Pros for Zone:
Protecting areas (if your DBs are slower and there is a speedy WR, this would make it easier)
No need to double cover a star WR
Easier to confuse the QB
Better suited if you have a strong DL and a slower secondary