I purchased Photoshop CS3 and absolutely love it - the power is mindblowing. I have never tried Fireworks, but I believe it's more geared for web design images.
I have to say Photoshop. I have never used Fireworks but I used Photshop when I go over to my friends house. If you make any thinh Photshop and you tried it will come out ok even if you have no skill at graphic like me.
Fireworks is more of a lite version of photoshop for designing graphics on the web. For example in photoshop you can save for web and devices, Fireworks saves it like that as normal.
Photoshop in my opinion is much better and more advanced giving you more tools needed for web design work.
I took a class with FireWorks at my school and it's "okay" but just "okay" when compared to Photoshop. However, just like any other program, it's about how used to it you are and how well you can use it. I've used Photoshop for awhile and I know quite a few things about it. If I spend some time in FireWorks I could get better with it just the same.