I don't know the break even point, I don't think you should worry about 1 or 2 pounds overall difference. It will make a difference, but it's so small that unless you are looking for top performance, ignore it. Plus, the clutch kit is normally weighed with the pressure plate. The pressure plate and flywheel should be considered 1 unit. A lighter flywheel with a heavier PP will cancel any lighter effect.
A heavier clutch will too, but it also effects the quality of shifting and transmission wear. Again, the differences are pretty small, so worry more about making sure it can has enough clamping force for your plan but not too much clamping force.