You don't have the option of choosing a land signal or the ship signal. When a ship is in port, the ship will turn off the 'ship' cell tower. As soon as the ship is out of land range, the ship cell service it turned on.
And again - you need to look at your specific itinerary and countries to make the best choice. I like mifi devices as I can have my laptop/phone and ipad all connected at the same time/same connection. A lot of crew prefer smartphones for Skype/facetime as they tend to be a faster connection wherever (whether their sim card or local wifi)