There is actually a 12 euro combined vaporetto plus bus ticket. The Alilaguna costs 15 euro spur of the moment, can take a long time if it makes a lot of stops from where you board. If it has already made numerous stops before your pick up point, it can be quite crowded or even impossible to board. It drops you at the ferry dock which is still 1/3 of a mile from the airport. The bus drops you right at the front door of the airport.
Check schedules from your boarding point to make an informed decision. www.actv.it
Yes it matters if you are connecting in Europe or flying to N.A. Was just there on Tuesday for a connection home via Zurich, walked in the door at 8:00 for a 9:45 am flight, was checked in by 8:20, through security by 8:30, sipping my first espresso by the gate at 8:40. Your mileage may vary.