Off roaders, should opt for a centermount progressive for smaller engines, and the centermount IDF/DRLA for larger. Choose the 40 IDF for <2000cc, and the 44 IDF for 2000cc and larger.
So - there you have it. All the ins and outs of carburetor and carb system options. Are you asking yourself why you've never gotten this kind of straight, down to earth, necessary information from the average VW parts supplier? My experience has been because they aren't typically all that knowledgeable about what they are selling you... so you should take the pre and post product support and knowledge into consideration when you are weighing a cost savings on your purchase... and also keep in mind that many parts suppliers don't supply installation and jetting instructions with your purchase! If you have a problem with installation and dialing in, you sure don't want to be on your own.