I would prefer a 7 day to a 5, but it depends on personal choice. If you have to travel much to get to the point of departure, you may have to add another day or 2 to the duration to account for this travel time. Many people get to the point of departure the day before in case there are any travel problems or to give themselves enough time buffer.
As for Inspiration vs Miracle, I'd go with the Miracle. Much newer ship, however prices on it seem to be among the highest in Carnival's fleet.