If I were intending to print the picture and wanted a slight edge in quality, I'd probably save the original as a png or perhaps as a raw (unprocessed) file. If I intended to simply store it for digital viewing, I'd save it as a jpg.

BUU is right in that if you convert a jpg to a png, you'll gain no quality edge and if you convert a png to a jpg, you may lose a small amount of quality, although I'm not visually discerning enough to notice.