It's a fair question, and the honest answer is that neither option is wrong. A taxi and a private transfer both put you in a car with a licensed driver heading to the same destination. Where they diverge is in certainty — over price, over waiting time, and over what happens if your plans change at the last minute.
What a Taxi Actually Offers
Valencia's taxis are plentiful, licensed, and queue in an organised rank directly outside the terminal. You don't need to book anything in advance, which is genuinely useful if your plans are last-minute or your flight itself was booked only days before travelling. The fare into the city is usually fixed or capped by regulation, so pricing surprises are rare. What you're giving up is control: you take whichever car reaches the front of the queue, with no way to request a particular vehicle size, and no one tracking your flight if it's delayed.
What a Private Transfer Actually Offers
A private transfer is arranged before you travel — you choose the vehicle, confirm the price, and a specific driver is assigned to your flight. That driver monitors your arrival time, so a delayed landing doesn't mean losing your place in a queue or paying more once you're finally through Arrivals. It costs a little more than the fare on a taxi meter, in exchange for removing every uncertain step between landing and getting in the car.