DPF Delete Pipe: This is a pipe that will delete the DPF only. This pipe will bolt directly in place of the DPF.
DPF/Cat Delete Pipes: This kit will delete the DPF/Cat and bolt directly into your factory exhaust. This pipe kit has the 2 bolt ball and socket flange on one end which bolt up to the down-pipe, and a slip joint on the other end which bolts to the DPF back portion of the exhaust. This kit is recommended over the DPF delete pipe alone as this kit is easier to install and gets better results.
Down Pipe Back Exhaust: These kits will replace everything from the down pipe, back. The first pipe in the system bolts to the 2 bolt ball and socket flange at the bottom of the down pipe and replaces the rest of the exhaust. This kit will delete you DPF, Cat, and remainder of the exhaust.
Turbo Back Exhaust: These kits will replace your entire exhaust system. This is the kit recommended for best results as it not only replaces everything that the down pipe kit does, but also the restrictive down pipe. While the down pipe looks big from the factory (4") it is actual double walled meaning there is another pipe inside that pipe which is squeezed down close to 3". All turbo back kits will use a 2 piece 4" down pipe as that is the largest diameter that will fit.
Most people go with a down pipe back kit because they are very easy to install and perform well. Also, some are scared to remove the stock down pipe because you have to cut it out meaning the factory down pipe can never be put back in place. To some this is a big deal, to others it is not as they would rather have the extra drop in EGT's.
As far as exhaust size goes, 4" will get the job done, but most really like the sound of the 5" kits due to the deeper tone. The 6.4L remains rather quiet even when straight piped so 5" without a muffler is very popular. If you want a deep sounding system without the turbo hiss, we recommend a 5" kit with a muffler. If all you are after is performance, go with a 4" straight piped kit.
Bungs vs. no bungs is another question that often comes up. If you are using an H&S or Spartan tuner you want to get a kit without bungs. The bungs are there to put the factory sensors in. Since the H&S and Spartan tuning disables those sensors, they are not needed. I have also heard that the SCT tuning no longer needs the bungs, but the sensors must remain plugged in under the truck and zip tied up out of the way. To me, it would probably just be easier to use a kit that has bungs at that point.
I hope this helps clarify a few things. Feel free to contact us if you have any questions.