I don't use either...I use Fireworks.
Photoshop is for more complicated image editing and for print production. Fireworks seems to understand what a web designer needs and all the tools are in just one application. Plus, FW gives you really precise pixel control and what I really like most is that if you draw an object using paths you get a very clear idea of how it's going to scale.
Even the great John Hicks/webdesigner claims he uses FW (http://www.hicksdesign.co.uk/).
That being said, if I could only pick one of them I'd go for Photoshop, but as it is, FW is my preference.