Check out a couple stills from Tiffen's 4K Diffusion Test. We'll save you the boring stuff as we have concluded that the Black Pro Mist and Soft FX combo is our favorite out of the dozens of filters that were tested. The Black Pro Mist provides a great base layer of diffusion while the Soft FX which is the Base optical resolution reduction effect while the Black Pro Mist which allows you to increase your control on practical and specular halation and contrast lowering effect in the image. That's a nice way of saying it produces a more organic look while maintaining detail in the overall image.
You can see the full demonstration here:
Here's one DP's review of the Black Pro Mist from Tiffen.
Here you can see a more detailed comparison between the 1/4 and the 1/8 Black Pro Mist.