Strength and Stiffness is quite important but vibration dampening is more important IMO.
Chasing light weight for gantry is a mistake unless you have high velocity/accel and fast directional change requirements.

Mass really helps when it comes to cutting and often a heavy gantry while going slower can cut a job faster than light gantry going fast due to much deeper DOC(with correct spindle), it will also give a better quality finish.