Well, DBU (dialog base unit) is the more recommended as it makes sure that the dialog will look the same for each user, same width, height, ect..
PIXEL however, allows the dialog to "adjust" depending on the users screen resolution, i.e. 1024x will make the dialog smaller compaired with 800x
An interesting arguement for using pixels might be because people who tend to use different resolutions may or may not do so because of their eyesight, perhaps they cant see the screen very well at 1024 or something.
On a 800x600 1 DBU = 2 PIX.
Eamonn.