I guess it would depend on where you feel the engine needs the extra oomph.
I personally feel the 1nz severely lacks in the high-end, so when I had my box, I used a DC Sports 4-1 ceramic header. I also do a considerable amount of highway driving, which led me to the purchase of a 4-1.
If we keep the xA, I will probably do the same, seeing as I still do more highway driving than city.
the MMW part is insanely boss though. I'd do some testing with both styles if you can and decide which one suits you best.