The Deck 6 cabins and the aft wrap Neptunes are SBs and SCs, (as opposed to the SAs on Deck 7) so will be cheaper. However, the cabin size on the aft wraps can be a bit smaller. The Neptunes midship on both Deck 6 and 7 are all the same size.
We have two SCs booked on N. Amsterdam for next year, and have stayed in the Deck 6 SBs before.