Here is how they stack up, and my own opinions on which is "best".
Bare flash. As I see it, to be used only when you have no other choice.
Stofen Omnibounce. It definitely helps even out the light on the subject, and reduces shadow intensity. I consider this an indoor unit. If used outdoors with no walls or ceilings to interact with, I feel the results would not be as effective, though likely still better than bare flash. A huge advantage of the Stofen is it's small size and fast, easy mounting on the flash. That alone can make it something that will be used rather than left in a closet at home.
Gary Fong Lightsphere. Hands down the "winner" as far as even, soft light on the subject and greatly reduced shadows. Disadvantages are it does "eat" light output, and is not exactly always easy to pack in a camera bag. Some people refer to it as a "piece of Tupperware". It certainly is not discreet, and adds some real weight to the flash. However, if you have a tilt and swivel head on your flash, it practically eliminates the need for a flash bracket. Again, like the Stofen, this is designed to use indoor walls and ceilings to bounce light, and is less effective outdoors. It "can" be used outside, but light power is being dispersed in all directions, and simply being lost and wasted.
Mini soft box. Somewhere between the bare flash and Stofen. Light is less harsh on the model than a bare flash, but not as soft and even as with the Stofen and Lightsphere. Shadow is also more distinct than with the Stofen. However, this is a true diffuser, with no reliance or design to bounce light in any way. Thus, light output level is not so severely compromised, and it is as effective outdoors as indoors. This unit would actually be my choice for outdoor use when using flash for FILL light. It folds completely flat and uses a velcro strap to secure the flaps of the soft box to the flash head. This was an $8 item on eBay, and for that price, it is worthwhile to have in my opinion, if only for outdoor fill light applications.
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