'Diffuser' is a pretty general term. What are you talking about exactly?
The Omni-bounce is meant to be used when you are bouncing the flash (off of the ceiling for example). It sends some of the light forward to fill in shadows that might be caused by the light that is being bounced. It also spreads it out in other directions, with the hope that it might bounce and come back.
A lot of people mistakenly believe that just putting something like this on the flash will soften the light by itself. To make a light softer, you can either increase the size of the source or move it closer to the subject. Sure, the Omni-bounce increases the size of the flash head, but only by a little bit....and it's not really any closer...so it won't really make the light softer. But it will rob you of some light.
The easiest way to make your flashed light softer, is to bounce it. Your 'light source' essentially becomes the surface that you are bouncing off of, which will probably be a lot larger than the flash head. In normal sized rooms, you can even point the flash up and behind you (try it).
Most flash accessories are made to help you, while you are already bouncing the light. A simple bounce card costs next to nothing and does basically the same thing.