Power. The B1600 is one stop more powerful than the B800, which is one stop more powerful than the B400. Flash duration will be affected by the increase of power, in that it gets shorter as you move up in flash model.

B800 is our most popular light as it is a good all around flash. If you need a lot of power, like shooting outside in bright sun, then the B1600 may be better. If you are wanting to mix with interior ambient lighting, the B400 may be a better fit, as it can go lower than the others.

If a broad range of power adjustment is required or desired (i.e. you shoot in dim light today, and groups on the beach tomorrow), then you may consider an Einstein640. http://www.paulcbuff.com/e640.php

I have only natural light (huge windows) - no ambient.

"Ambient" light is light that is already there, be it natural or artificial. Light coming in through a window would also be ambient light you could use along with a studio flash.

If you wanted to use the natural light, I would say go with the B400. Otherwise, the B800 would be a better choice.

You will definitely need a modifier, and softboxes are a very popular choice, often purchased with a grid. I would suggest a 32x40 as an all around.

Alternatively, you go with our PLM system which mimics octaboxes very similarly, except you cannot grid them (though some configurations would not require grids). For your uses, I would suggest a 51" or 64" soft silver, 8mm style with a white front diffusion fabric. They are smaller, lighter, less expensive and easier to manage.