I reckon that GamerPrinter has it about right. Each type of app runs on a methodology. Photoshop can do everything but its geared up for drawing individual pixels. The vector apps are good at some things where a CAD like look is what is required. Vector apps have advantages particularly with scaling but have issues with full color photo images but you might find that they can do some limited ability with photos. Icon based programs can be quite easy to use and quick to get something up but they will not handle either pixel based operations like flood fills or do vectored operations but again you will find that these apps partially cover for some of those limitations. Then multiply all that again for 3D.

My recommendation is to become good with one of each of them and know what is best of each. There is no point in trying to force one app to do a job that another is geared up for. And don't forget a good pen & scanner or digital camera in that tool set too. I can draw curvy thin lines with a pen faster than any bit of software if you have enough of them then use a pen and I can take a photo of a tree faster than I can draw it.

I tend to draw the outline with a pen, scan it in, use a raster app like PSP, Gimp or Photoshop to touch up, fill and get basics in then import into my icon based app and add extras like tables, chests, people, buildings or whatever. Some people would take it back into a raster process to add effects and shadows, which although it looks great and turns a map into a piece of art, its just not my bag and no criticism from me for anybody who does.

Most apps can change between formats though I would say that its a lot harder to go from raster to vector than it is to from vector to raster. I think this is the reason why raster apps tend to dominate.