Carnival transfers will be $15 per/person, $75 total. 2 taxis will be around $60 including tip. I would do the taxi. They are plentiful at the pier. Grab a porter and have the porters grab one for you. It makes things much easier.
If you can get a van taxi then the cost would be less.
If you use a taxi you will avoid the hassle of waiting for the bus to fill and also the rush at the airport as all the people unload their luggage. Also, with a taxi it is easier to use curbside check-in.