Fundamentally speaking, diffusion panels and softboxes serve the same purpose: creating a larger source and thus, softer overall light. However, in practice, they can have different effects on your subject and can also be easier or harder to work with depending on your needs and setup. This great video will show you their advantages and disadvantages to help you decide which is right for your work.
Coming to you from Daniel Norton with Adorama TV, this excellent video discusses the differences between diffusion panels and softboxes. Softboxes are one of the first modifiers most photographers try, and they are a cheap and relatively versatile tool that you will probably continue to use throughout your career. A diffusion panel, on the other hand, is simply a piece of translucent fabric fitted to a frame. You then aim your light at it and shoot through it to soften it. These can be a little easier to work with if you do not want to deal with different speed ring adapters and want to be able to quickly move lights around and make adjustments without changing out for different sizes and the like. Both have pros and cons; check out the video above for the full rundown from Norton.