JPEG does a better job with realistic images such as scanned photographs. Netscape can handle JPEGs, and support has arrived in many Mosaic flavors as well. To allow those with other browsers to view your JPEGs, wrap a "normal" link to the image around the inline image so the user can download the image for display in an external viewer.
GIF does a better job with crisp, sharp images, such as those typically used to construct buttons, graphs and the like. All browsers that can display graphics at all can display GIFs inline.