From my E60 and F10 535xi's, (outside of hes. from the N54) I think the N55 was a bit nicer of an engine. The transmission of course comes into as a differentiating factor. The N55 definitely helped with the turbo lag and it would drive more smoothly. Out of the two engines, I think I liked th N55 better--but the F10 is a bigger boat than the E60.
Auf Wiedersehen 12 550xi