You and your location to the model.
I have used different diffusion methods both ON and OFF camera. One of the issues that you have to look at is the distance the flash is to the model and the distance the model is to the background.
The farther away you are from the Model the the more contrast you will get on their face and the harsher the shadow will be on a close background. This light fall off need to be managed. So the close you get the less fall off and the softer the light. Also the closer you get the the model the greater the ratio relationship is between you models meter reading and the background. Say you want a darker background, well don't move back move forward.
But, no matter what diffusion you are using you will still get harsh shadows depending on how far the flash is from the model. Its the distance the light is from the model not the camera from the model that is important here.
Now Say you are shooting a model and you don't want harsh shadows on the model then move closer. This will create softer shadows on the model and on the background behind the model. If you have off camera flash then move the flash closer if you are worried about perspective control.
You can diffuse with anything translucent, its the amount of light passing through it and the distance from the model that will make the difference.
So, I would before spending more money on equipment, look at some basic photographic lighting principles first and test, test, test.