I think it is largely a matter of preference, but I have always used Fireworks and find Image Ready counter intuitive, clunky, slow (though FireWirks is slow also), and lacking in features. Fireworks MX 2004 promises drastic performance increases, so speed will not be an issue (I hope).
I use Macromedia products for all of my web production, so FireWorks integrates much better. If you use Adobe GoLive, the maybe ImageReady will work better.
I still use Photoshop for certain tasks though.