photoshop is a lil more versatile. i never used fireworks, but macromedia products are a little more simple then the photoshop series
It's kind of like comparing apples and oranges. They are different programs designed for different purposes. Always use the best tool for the job at hand. The only way to find out which program is best for a given job, is to try them both out and see what works for you.
To put it simple: Photoshop > Fireworks. I have both programs at the moment, never touched Fireworks because it seems so basic and novelty compared to the industry standard Photoshop.