Be the first to rate this answer!
(I tried to reply via my Android, but might've got lost in the ether).
So, yes, the Interislander is the larger service, with three ferries crossing Cook Strait, and is usually a cheaper for taking vehicles across. Bluebridge with two ferries, has a cheaper fare structure for foot passengers, and they are also advertising free wi-fi. But you should check their websites, as there may be specials depending on the time of year.