Fireworks is definitely better at exporting PNG files in terms of quality and file size.
I use it in conjunction with
PNG Gauntlet to compress more (no loss of quality).
If you compress all the 64 default x-cart PNG images (skin/common_files/images) using the PNG Gauntlet tool, you'll save 79k overall !
Here's a bunch of useful, free Fireworks extensions -
Download 