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.
1999 Techno Violet Metallic BMW M3
2005 Audi B6 S4 quattro 6MT
2004 BMW 330i
1986 BMW 535i