Honestly I had an N54 for 2 years and sadly it got lemoned. The dealer couldn't fix the car for 2 months it sat at the dealer. It had no performance modifications except for an Injen DCI. I had problem after problem with that car since the day I bought it.
My N55 with JB4, Injen, and a CP. Has been running smoothly and I'm the type of person who doesn't care too much about numbers. So the reliability of my N55 is good for me.
2011 Alpine White E92 335i M-Sport N55 Engine