The #1 rule is always to find as light tools as possible to reduce the risk of work related disorders linked to the usage of power tools. Weight is one of several factors that makes up the overall ergonomics of a power tool.
There are only a few occasions when a heavier tool has a positive effect on the ergonomics of the user: grinding and chipping on horizontal surfaces are two examples of that.
The benefits with heavy tools are lower vibration emission and, on occasion, less need to apply feed force.
The disadvantage is the increased risk of work-related musculoskeletal disorders due to the forces the weight applies to the hand-arm-shoulder system.
For all jobs where no particular feed force is required, the tool weight should be kept to a minimum. If the job also includes bad working postures and a lot of repetitive motions it’s even more important to make sure that the tool is light.
Weight is like any other external force and it should be kept as low as possible at all times.
Material removal tools require a certain amount of feed force to remove the material.
If the workplace enables the user to work below shoulder height on a fairly horizontal surface, the weight of the tool will contribute to the feed force and the user doesn’t have to push so much. The total force applied by the user will then be lower when the tool is heavier.
On the contrary, if the work is above shoulder height the user must first overcome the weight of the tool and then apply the feed force. That is a situation you should avoid as at all costs!
Heavy on the metal but not on you