A range of independent PNG images has the advantage that each image can be differently tuned so that is remains recognizable.

An arbitrary SVG that looks good at 256x256 pixels may not be suitable if rendered at 24x24: detail is lost but you have no choice of which detail and cannot adjust other details to suit. You can work out an SVG that works at various sizes... Inkscape, for example, has the Icon Preview view so that you can see what you will get at various sizes as you edit the SVG.