Where each one drops you
ADO runs from the terminal to bus stations on a fixed timetable. If your hotel is a few blocks from one, it is a short walk. If you are staying on the Tulum beach road, in a gated development like Puerto Aventuras, or anywhere away from the station, the bus is the first of two trips, and you will handle the second with luggage in a town you do not know yet.
Add that second leg to the ticket price before you compare.
For Playa del Carmen and Tulum there is a bus station in town. For the Tulum hotel zone, Puerto Aventuras or Akumal, the bus leaves you some distance from your hotel; a private transfer ends at its door.
What happens if your flight lands late
A bus leaves on schedule. If your flight lands late, you wait for the next departure, and at some hours that wait is long. With a private transfer we track your flight: a delay moves your pickup instead of costing you a ticket, and 90 minutes of waiting are included in the fare.
It matters most on late-night arrivals.
When the bus makes sense and when a private transfer does
ADO works well for one or two travelers with light luggage, a daytime arrival and a hotel near a bus station. It is a scheduled service, with timetables on ado.com.mx.
A private transfer fits better when you are traveling as a group, your hotel is far from a station, you have young children (child seats are free on request and installed before pickup) or your flight lands after midnight.