I think it depends on what you find easier. I use fireworks for layouts and general web graphic creation because I find FW to be far easier than PS in terms of basic web work and just easier to use in general.
Fireworks does the job for me but I'm sure others would say the same about Photoshop. I use PS for complex, layered images like blending photos/masking etc.
I do not think there is any distinct advantage to either if you are equally comfortable with each.
What's most important, IMO, is a solid foundation in CSS/XHTML.