Actually, they work very well for soup. One can have a very good barley, beef and mushroom in 15-20m for example. I know one does not get the depth of flavor from a looooong simmer but it's surely better than opening a can. Where cooking fuels are expensive people have been making soups and stews in pressure cookers for years. And they are invaluable for those who get home at 6 and want to have a decent supper before 10. More of us here in the US should be familiar with all they can do.

I've never used a pressure cooker for soups, but it might work. But they're designed for speed and I think what you could lose is a well-developed flavor base you get from slow sauteeing of onions, celery, carrots, etc.

Slow cookers work well for those who can't be home watching the simmering pot on a cooker. A pressure cooker is a great asset when in a hurry and want a decent soup in a short time. There are lots of good recipes out there for both.

Neither. And they are kind of contradictory. A pressure cooker speeds up cooking, and a slow cooker, well... A soup pot on a kitchen burner simmering away, uncovered is the best method.