I think it's pretty simple to boil down...
If you're going automatic, go N55. Period
. EVERYONE raves about how the DCT is better than the step. (I have a N54 step)
If you're going stick, it'll be up to you and how fast you plan on tuning. If you're looking for quick, guaranteed power, go N54. If you are willing to wait and see what develops, go N55.