If I remember correctly, most of the 5 night Bahamas cruises out right now are the ones with 2 stops at Castaway. Which is amazing, but my jaw dropped when I looked at the rate for an inside stateroom for a double dip! But you are getting your money's worth I'm sure.
But to OP, we did a 4 night as our first cruise on the Disney Wonder and loved it. Nassau, Castaway Cay and then a day at sea. A pretty quick vacation, but we also felt rested at not rushed to do everything we saw. 7 nights have a lot to offer. We went on a 7 night on another line and had a blast. If it's your first, I would definitely do the 4 night with the day at sea. There's so much going on and it's so relaxing to look out and see nothing but water.