Originally Posted by claykin
The N55 is only as reliable as the one BMW built for you. And how well you take care of it.
Overall the N54 is a more complicated design. It also has some higher end components such as the piezo injectors and true dual turbos. N55 is what most would consider a cost reduced version of the N54.
The N54 also uses forged pistons, not cast ones, among some other things I can't recall this late at night. If you were going to go nuts out and tune your car significantly, I'd pick an N54 engine over the N55.
OP -If you're just a daily driver with ambition, it's not going to matter to you.