I was stuck in the same dilemma Hurricane. I went with the SL55 but it was a damn tough decision. I still dont know if I made the right one, but I will be installing the SL55 setup soon. If I dont like it I will sell it for a NW dual.

I paid $550 for the y pipe, airboxes, tvt tubes, couplers, and mounting brackets. This is $250 cheaper than NW dual. Also, should be minimal SC whine compared to NW dual.

If SC whine, maximum performance sound good to you, (at a very hefty price) get the NW dual.
If minimal SC whine, near maximum performance and stock look would be better (along with being considerably cheaper), go with SL55.