All these diffusers - Stofen, Fong, Lambency etc - work in the same way. They redirect most of the light all around - above, to the sides and behind - and also forward. All the light that is bounced around needs a reflective surface like ceiling and walls to provide any fill-in light which does virtually all the softening.
With the Stofen, the light coming directly from the head itself is not softened at all because the area of the Stofen is still small. The Fong is bigger and this gives some softening of direct light, but not a huge amount especially at distance.
Basically, the room you shoot in has far, far more effect on the final result than there is any difference between the diffusers individually, unless you are shooting close, when the bigger diffusers are softer.
If you shoot outside when it's dark you will see the effect of the direct light only, and it's not massively different to flash without any diffuser - unless you are close, when the size of the diffuser is relatively bigger. Then again, if you shoot in a small light coloured room, the effect is dramatic with lovely soft light bouncing in from all around, plus the direct element lightening faces and putting a sparkle in the eyes. It looks great.
By the same token, a bounce card does exactly the same thing (see here
www.abetterbouncecard.com ) and so does the little pull-up hi-lite card built in to many hot-shoe guns. I don't think the 550EX has one, but you can use the diffuser wide panel pulled out but not flopped down and get a similar result.
I use a Stofen because in the right room it works really well and is so easy (and you don't look so stoopid). When the room is more difficult, or I just want to look a little daft, I use a Lumiquest Quik Bounce. It does everything pretty well - soft-fill, bounce-fill, vertical or horizontal shooting. It is also more efficient by not blasting light out of the back and sides which is mostly not much help - it's the ceiling bounce that really makes this flash technique work. This fella
http://www.warehouseexpress.com/buy-lumiquest-quik-bounce/p1031217