i read this like 2 days ago on a photoshop for dummies book book i got. Its camparing the two file formats for web graphics
JPG: Use JPEG for photos. Remember to resize the photo so that it fits
on a Web page. When selecting a Quality setting, you need to balance
image appearance with file size. A smaller file downloads (and displays
in a Web browser) faster, but a larger file generally looks better. Reduce
the Quality setting until just before the image doesn’t look great and
you’ve hit the sweet spot — the compromise between file size and image
quality.
PNG: PNG comes in two types: PNG-8 (which is a substitute for GIF) and PNG-24 (which is a substitute for JPEG). PNG has a couple of advantages for Web designers, such as support for transparency, but not all Web browsers can display PNG graphics. Generally speaking, it’s safer to use JPEG and GIF.