Either one will work fine if you want more sound. The aftermarket systems like Stillen, ARK, etc. are engineered to maintain OEM scavenging, plus they are made to bolt on so you can DIY. If you want a custom set up you will need to ask yourself what are you wanting to accomplish with your car, as you have several possibilities as far as design, pipe size, etc. With the 5.6 you have a front Y-pipe assembly that has a secondary set of cats (3.7 does not have the extra cats) a dual resonator assembly and factory reverse flow mufflers (both motors share). Before I got my mandrel bent set up, I had a muffler shop delete the secondary cats and replace the rest of the Y-pipe and resonators with an X-pipe and straight pipe. Here's a good photo of what that looks like:
This is factory 2" pipe. You are deleting a lot of restriction by simply doing this, and it's the cheapest option. The resonators alone choke down to 1-3/4" ID. For a stock tune, this will be plenty good enough, and actually the factory mufflers shown will sound awesome with this set up. With aftermarket muffs like Flowmaster or Magnaflow it'll even sound better. If you decide to do a custom tune you'll want to go with bigger pipes.
I payed 30k and had it sent to infiniti for inspection. The service tech said the car had no issues. 
