"What tool should I use?"
The context is usually framed within the hope of a firm decision to use one application over another. Illustrator or Freehand? Illustrator or Inkscape? Illustrator or Photoshop? Illustrator or InDesign? Illustrator or Fireworks? Oh the choices are endless, aren't they?
In general, I try to nudge people in the general direction of using the application they are more familiar with. Whatever tool you can use faster without having to think too much about it is the one you should use. If Photoshop seems natural to you, then go with it. In my case it's Illustrator.
Why do I offer this advice?
I offer it because I believe it's a matter of using a tool for what it's best suited for. In my opinion, the most powerful aspect of using a computer is the ability to iterate on a concept. To generate as many variations of a concept as you can.
But we must not forget that before you start iterating on an idea, you have to come up with it in the first place. And a pencil is FAR more powerful of a tool for that. I usually differentiate the two in the following way:
You limit yourself (and your creativity) if you try to explore within the confines of any computer application.
I offer the following example: Here's a logo I was recently working on:
The sketch on the left is one of about 30 or 40 that I doodled, over the course of about 3 days. It took a lot of time. The three vector versions on the right (in blue) took me about 3 minutes. So it was only after I had spent time exploring with basic concepts that I was ready to start iterating on those concepts.
And here's why I use Illustrator. Because I don't have to think about HOW to iterate. I'm fast enough to try multiple iterations without slowing down. And that's why I say if you're familiar with any application -- be it Photoshop, Fireworks, etc -- USE it. Just use it for the right thing. Use your pencil to explore and then use your computer to iterate.