In stock form, I would take the N55 over the N54. It's more refined, and I found that the torque comes in sooner. If I had plans to modify the car/engine, I would take the N54. More tuning potential, more aftermarket support. I like the N20. I think it's a great engine with lots of potential. Would be cool in a 128i.
Past: 1999 Techno Violet Metallic M3
Present: 2012 135i DCT Dinan S2