Better than weight, if you can estimate/calculate the Moment of Inertia, even better.

Also, don't forget to pay attention to the belt stiffness, that costed us a couple of fights on Robogames 2008, the belts were too stiff and the motor didn't have enough start-up torque. The problem was solved switching from 3L to 2L belts, increasing the transmission ratio - was almost 1:1! - and using a lower KV motor (2300Kv to 1200Kv). With those changes and some re-design we managed to replace the Aluminium drum for a Stainless Steel one, even more MOI.

Cheers,
Eduardo