The best use of these flashes is for "bounce flash" or indirect lighting. Where the light is bounced off a rear wall or a ceailing. Notice that the light while still attached to the camera can be aimed up or even backwards.
If you want to take the speedlight off the camera nad place it on a stand then you are doing astyle of studio work that is not "fast moving" With a light on a stand your subject has to be cooperative so in my opinion you don't need automation, you have time to think and take a test shot or even to experiment with a stand-in subject Given that, I think you can use MUCH cheaper lights in studio. You can buy a good used Vivitar flash for about $40 that has more light output as the SB600 but lacks the SB600's automation. But if used on-camera automation is very nice to have.
The Sb400 seems good utill you start thinking about vertical shots. How can you bounce off the ceiling with out a swivel? Y'd need a folding type flash bracket and a sync cord. Better to just buy a SB600 if you plan on doing many verticals