SVG is scalable, if you have a vector-graphic that is a clear advantage. For pixel-graphics PNG is better. A downside is, that the Internet Explorer supports SVG only with the coming version 9 (before with plugin). Mobile browsers may also have limited support for SVG.
I would say PNG simply for the fact it seems to be a more accepted format than SVG.
I would say PNG simply for the fact it seems to be a more accepted format than SVG.