Hard to compare the 2jz and the n54.
The n54 just came out.
If your talking about right now (11/16/07) which is a better engine. I would say the 2jz.
But only time will tell what the n54 is capable of.
Previous - E90 335i, E82 135i
Cuurent Lexus: Ultra White RCF, Infrared RCF
Ferrari: 575M SA, 458 Italia
Rolls Royce: Phantom Porsche: 1973 911E
BMW: F30 335i, E89 Z4 3.5SI