Generally speaking (especially newbies, not long-time cruisers who just sail to be on the ship) what sets a 7-night cruise apart from a 3+4 B2B Bahamas cruise is the ports. A lot of people stay on board the ship in Nassau, making it effectively a cruise to
Castaway Cay on each cruise segment. Eastern / Western / Southern… cruises stop at 2-3 ports + Castaway Cay, leaving you 1-2 ship days.
Food - On B2B, you will repeat the standard menu in each of the 3 MDR, with one additional menu (?Pirate). The extra nights on a 7-night will each be a different menu (served in all MDRs that night).
Palo - if you want to try this and are not high in loyalty status, you will have 2 better opportunities to get a table (the first night of the cruise this does not book as quickly. You could miss the first night of either cruise in a B2B and still dine off the menu on the other cruise)
Yes, the shows, movies, activities are essentially the same; trivia questions can be the same for a season, it seems. I do not usually find I am having to choose between 2 activities in a time slot, so a second cruise is pretty much a repeat that way.
If you need the ship’s offerings to be new entertainment everyday, a B2B could be a let down.
If you want time together as a family, chilling and relaxing with the possibility of 2 days at the beach (Castaway Cay) within a week, it’s a nice way to go.
Edit:
(Whoops; not really your question, went off on tangent comparing B2B with 7-night).
So, yes, mostly the same; well, very much so.