I've got both, and for designing graphics from scratch FireworksMX is used 99.9% of the time. Easier and quicker to use, better features, and a hell of a lot better ouput quality and optimization. That's MY opinion.
I paid full price for both, and have pretty much written off the expense for photoshop as a total business loss. I have yet to see ANYTHING that would justify photoshop's cost or learning curve. It takes five times as much effort and time to do what I can accomplish with Fireworks with ease in just minutes.
Bottom line is make the choice for getting the job done with the least amount of time and effort. I'd suggest trying both and then sticking with the one YOU are comfortable with. Opinions be damned!
