Price: meter or fixed
A street taxi runs on the meter, so the price depends on traffic and the route the driver takes. Overcharging and longer routes are among the most common complaints the municipality receives about Istanbul taxis (source). A pre-booked transfer has a fixed price per car that you know before you travel. Tolls and parking are included.
Waiting: queue or name sign
At the taxi rank you wait in line, sometimes for a long time after several flights land together. With a transfer your driver tracks your flight and waits in the arrivals hall with your name sign. With ITB Transfer the first 60 minutes after landing are free, so a long passport queue or a late bag costs nothing extra.
Luggage and groups
A yellow taxi takes up to four passengers and limited luggage. With a transfer you choose the car that fits: a sedan for up to 3, a minivan for up to 7, a Mercedes V-Class for up to 6 or a minibus for up to 16. One price per car, not per person.
Language and safety
You do not need to explain your hotel address in Turkish: it is already in your booking. You travel with a professional driver who knows where you are going, and you pay the agreed price at the end.
When a taxi still makes sense
If you land at a quiet time, travel light and alone, and your hotel is close, a taxi from the rank can be fine. For families, groups, late arrivals and anyone who wants to know the price in advance, booking ahead is simpler.