It depends on what you want to do. Both have pretty good clarity. I would go with .jpg for large image to save on transfer time. They are compressed so less bytes have to transfer. I'm a big .gif fan myself. It's a memory thing. Gif only requires the memory equal to it's size. Jpg needs to be "blown up." It has a tendency to crash browsers running on systems will small memories.
Eric Bauersachs writes:
GIF has maximum 256 colors, JPG is always True Color.
JPG is normally better for photos or images with lots of colors.
GIF uses compression which doesn't loose any information. Loading/Saving JPGs several times will degrade image quality.
For screenshots you MUST use GIF files to keep lines straight.