neuro, do you suggest getting the softbox for the Speedlite AND bouncing it off the ceiling or one or the other?
Bouncing off the ceiling will be better than a softbox. The largest softbox you can mount directly to the Speedlite on-camera isn't really very big, usually not more than about 8"x10". I have a Lumiquest Softbox III that I use occasionally, but usually with the flash mounted on a flash bracket and connected with an off-camera cord. Even then, you need to get close to the subject for soft light.
The StoFen diffuser isn't a softbox, just a little translucent cover for the flash head. I use it when I bounce the flash - it throws a small amount of direct light at the subject, which is good for fill and catchlights, while letting most of the light bounce off the ceiling. FWIW, the 580EX II has a little pull-out bounce card that does something similar.
For a softbox, the bigger the better. I do have a couple of Lastolite 24" Ezyboxes that I use with 430EX II's (triggered by PocketWizards), and an even bigger 48" octabox that I use with a monolight (studio flash). But I wouldn't go there striaght off...