How the airport taxi works
Taxis in Dubai are metered, regulated and generally fine. At the airport you join the rank outside arrivals, the flag fall is higher than a street pick-up, and the meter then counts distance and time. That last word is the one that matters: time. The same route costs one thing at 4 am and another thing behind an accident on Sheikh Zayed Road.
At peak arrival banks the queue itself is the hidden cost. Several wide-body flights land inside the same half hour, and the rank is sized for an average evening, not for that.
How the fixed transfer works
A private transfer inverts the deal: the price is agreed before the trip and printed on the booking. The chauffeur meets you inside arrivals with a name board, and the office watches the flight number and moves the pick-up to about 45 minutes after the actual landing, roughly when you walk out into arrivals. The free hour of waiting starts from then, not when the wheels touch down.
Traffic, the toll gates on the way, parking and the airport pick-up fee are the operator's problem, not yours. If the drive takes longer, the price does not move.
When the taxi is the right answer
Travelling alone with a cabin bag, landing at a quiet hour, going somewhere close: take the taxi. It is there, it is metered, and it costs less than a pre-booked chauffeur.
When the transfer wins
Three or more people, real luggage, a child seat, a late-night landing, a meeting you cannot be late for, or an Abu Dhabi leg. A transfer costs more than the meter would, and we will not pretend otherwise. What the difference buys is a chauffeur waiting inside arrivals, a car sized for your party and your bags, no queue at the rank, and a price settled before you fly.
The honest summary: a taxi is a good product with a variable price, and usually the cheaper one. A transfer is the same drive with the variability removed. You are paying to delete the question mark.