The N54 is better, but then again I'm biased....
I think the biggest positve for the N55 will be lessons learned from the N54. You probably won't see all the fuel pump, spark plug and injector issues that the N54 has had.
Tuners will have their work cut out for them (again), but if they can get useable power out of the N55 it will most likely trump the N54 due to valvetronic. This of course is yet to be seen.
The new M3 will be interesting seeing that it will have Twin Turbos.
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2011 Mineral White M3 ZCP - Opticoat, Ventureshield, some CF, B&B exhaust, AFE Intake Vorsteiner VFF 101s Ohlins
2014 Mineral White 328d Xdrive Wagon - Opticoat, Ventureshield, B8s, H&R, Apex SM-10s Hawk Pads
2021 718 Cayman GT4 - Fire extinguiser