It all depends on when are you travelling, how many people there are and what your budget is. Ljubljana is en route between the airport and Maribor, so changing buses in Ljubljana does not mean travelling in the wrong direction. You could also change to a train, the train and bus stations are next to each other. The cost of the entire trip by public transport would be around €15-20 per person, depending on the connection you chose. A shuttle service such as GoOpti, booked a few weeks in advance, would set you back about the same amount of money per person.