[zopticity]

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.

[sp332]

IE 10 is the current version. It has already taken about 20% of IE 9's market share.

[twistedpair]

Older Android browsers lacked SVG support too. Bug report said it was to save a few MB off binary. Come on man.

Webkit acceleration is getting much better, and for those ancient IE browsers, check out RaphaelJs which translates them into MS approved VML.