Sounds pretty simple...
The N54 has had time to adapt and optimize. The N55 simply needed the time to adapt and optimize. It could end up a wash, and it could end up in one engine squeaking out the other, but it probably won't be by a wide margin. Whether one has better internals for bigger turbos than the other will probably be hard to say as well. They both seem to be doing what they should be doing. Who loses?
About the only thing that i've heard enough about to give pause is the DCT handling the upper end of the power potential.