[sp332]

Not unless you have only 256 colors on your desktop.

Edit: Even if you're OK with indexed color, the indexed PNG will still be smaller.

[tomlin]

It usually doesn't matter. If you have photo-like data in your screenshot, you're better off going with the JPEG.

EDIT: You're probably right about the indexed PNG.

[sp332]

I don't have a .jpg optimizer, but a PNG screenshot run through pngcrush has been consistently smaller than a .jpg compressed to the wrong side of tolerable for me.