I'd recommend Fireworks if you want to iterate the whole layout and if you don't need to invest too much time polishing the graphic style of UI elements. Fireworks has a lot more direct manipulation of the canvas elements.
Photoshop on the other hand has more and better graphics manipulation tools: color pickers blending options and effects. It you need to come up with a carefully crafted graphic style, use Photoshop.
Photoshop on the other hand has more and better graphics manipulation tools: color pickers blending options and effects. It you need to come up with a carefully crafted graphic style, use Photoshop.
I use both, depends on the task I need to do.