Photoshop and Illustrator compared, both expose nearly the same possibilities to create icons except that Illustrator uses a vector based system. In which situations is which product more reasonable and when should I prefer Illustrator over Photoshop or vice versa?
You answered the question in your question. Use Illustrator when you need to leverage the Vector tools. Use PhotoShop when you need to leverage the raster tools.
Or switch to Fireworks and have both at your disposal.
There's no specific answer to this...it all depends on many many factors...what kind of icons you're making, the visual style, what medium they'll be used in, what kind of variations are required, etc, etc.
In the end, most designers have a toolbox with all sorts of software and likely would end up using both for a lot of their work.