http://www.webdesignerwall.com/general/fireworks-vs-photoshop-compression/

"Photoshop is alway my first choice for designing—from image
editing to designing mock-ups. But, did you know that Fireworks is way
better than Photoshop in term of image compression? I’m not a software
engineer, I can’t explain why Fireworks can compress better. But I can
prove it to you by showing a series of experiments I did. "