Urd cause you are sc so the engine can breathe easier meaning more power, The engine is an air compressor so you have to be able to get rid of the air as fast as you suck or push it in. The URD doesnt have any crazy bends like the DT but does come in 2 peices and if you are lifted you will have to go to a muffler shop to have it extended.
OK, I understand the concept of how a supercharger/turbocharger works. The DT does have an extra bend in it and the "resonators" but I cant see it being much more restrictive than the URD. The resonators are basically jsut pass through so shouldnt cause much of a restriction, same with that extra bend. On top of that either of them will flow MUCH better than the stock headers, Y pipe, and cats...
Either way I can see both of them being a big improvement over the stock Y pipe and cats.