BADI or user exit, I don't think it is a choice between the two. Both serve the same purpose, add some custom logic to the standard logic. It depends on your requirement, the point when the user exit or BADI is called, information that is available to you in that user exit/BADI and information that you can change in that user exit/BADI.
I don't think there is a straight answer. Implementation wise, BADIs are futuristic user exits, in that they are object oriented. But other than that basic difference, you have to look at your requirement and the other factors that influence this decision as mentioned above.
