Tanks:
More space for bedding
Gerbils can't kick the bedding out!
Nicer for viewing.
Cages:
Some say they have more ventilation but as long has the tank has a mesh lid then the ventilation is fine. Gerbilariums have cage tops and are great also. I would say that tanks and gerbilariums are superior to cages. I have a tank with shelves called 'Critter's Choice'.
Here is a great video on tank and cage choices:
