By and large, they are the same as far as hardware specifications go. We don't sell the G550JK, so I don't have any firsthand experience to compare with the N550JK (the latter of which we do sell), but from what I see, by and large, the differences are the audio setup and aesthetics.
There are probably other dissimilarities, like the trackpad and/or keyboard feeling different, different cooling system, etc. Hopefully someone with personal experience with the G550JK can chime in.