Using "pixels" will give you finer control over the absolute dimensions of your dialog, in relationship to graphic elements.
Using "dbu" will help make sure that buttons and lists are scaled proportionally to the user's font size preference -- if they are visually impaired or just prefer larger dialog text. "dbu" will make sure that text doesn't get cut off because the buttons and objects are too small for, or overlap, the text.
You decide if your dialog can scale well to text, or if it's mostly image/graphics based.
