Sheet Metal vs Tube Drivetrain

[s5511]

Our team has recently obtained a waterjetting sponsor and would like to build a new chassis over the summer. We are currently debating between building a full sheet metal chassis or a chassis made out of aluminum tubing (probably 2 * 1). We have noticed that a lot of the best teams, like 254/118 have used aluminum tubing in the past.
What are the pros/cons to using each on your drivetrain?

Also, we were wondering why many teams use chain on their drivetrains, as we have always used belts.
What are the pros/cons to using belts vs chains on your drivetrain?