spinning disc
- more weight to the outside = greater kinetic energy
- Greater kinetic energy = longer spin up time

bar
- Can make the material very thick and there is little time for an opponent to hit the bar when rotating so it can be made stronger for the same amount of material
- Less material further out (assuming a uniform bar) = Less kinetic energy

The sums involving kinetic energy assume the same rpm of each design. At the end of the day you want a big mass, far from the center of rotation spinning as fast as possible.

You also have to consider vert vs horiz. Horizontal weapons generally can be made heavier as the gyroscopic forces involved act to keep the robot horizontal. Vertical weapons have the issues dealing with gyroscopic forces. If you want a good visual example then watch the middleweight final from extreme 2 when typhoon hit 259.

Vert weapons have the bonus that they react against the arena floor so more of the energy available goes into the opponent instead of being used to knock your machine away.

There's no real one answer beats all. Some are good and some are bad, usually when you do some basic sums it becomes apparent why. You can use the following calculator to get a rough idea of the kinetic energy involved.

http://www.teamcosmos.com/ke/ke.shtml

For reference, hornet 2 had a max of around 2kJ, I believe 360 is around the 5kJ mark and typhoon 2 could store around 40kJ at full speed. Last year boner had around 0.7kJ. More energy isn't always better.