When you do the B2B, you do get credit for two cruises for the
Castaway Club, if that matters to you any. And you have the same menus, etc. which is good if you liked the offerings. If you didn't, you can skip dinner in the dining room. Since you have seen the shows on the first sailing, there isn't as much rush to see them on the second sailing.
On the B2B, you don't lose much time on the day you are back at PC, but you are duplicating a lot of events, like the menus, sail-a-way, till we meet again, etc. You do have some time when you board the second sailing that the cruise is not that crowded and you can get some great pictures not filled with people, or ride the Duck and enjoy the pools while they are not crowded at all. You also get the two stops at CC, which is so very nice.
We have done both the B2B on the Dream and been on the 7 night Fantasy. I am glad we did both, but if I had a choice, I would do the 7-night Fantasy for sure. I loved

the Satellite Falls area on the Fantasy, which is not on the Dream.
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