The newest isnt always the greatest. That goes for both tuning potential greatest and daily driving consumer greatness.
The N55 seems to be and have less tuning potential than the N54 because of the valvetronic, and of course it going to come with some tough anti-tuner hardware/software mix that will most likely keep tuning power levels at bay.
For everyday driving, the N55 has the stuff to make it really good. If and how much better than the N54 we will have to see, tq coming in 500rpm early is going to be hard for alot of people to see a difference in there dail driving, so we will have to see, i expect to test drive one as soon as it comes out at my local dealer.
Tuning wise, the N54 is going to be the better choice. since there is more R&D which is 3 years into it if not longer which is major.
All we can do is speculate but tuning wise things are really leaning on the N54 side.
Daily driver and out of the box wise, the N55 has the right stuff but if it will be better we will have to see.