These are two different services, and confusing them leads to expecting the wrong price.
A one-way transfer is a single journey: airport to hotel, hotel to airport, or between two cities. The driver drops you and the job ends. If your arrival time is fixed and you do not need a car during the day, this is the right one.
With a car for the day, the vehicle and driver stay with you throughout. You get out where you like, look around and come back; the driver waits, parks and takes you to the next stop. On shopping days, when more than one district is being visited, or when travelling with a small child, the difference is large: you are not hunting for a car at every stop.
Which suits you usually comes down to this: more than two stops in the day means a car for the day; otherwise a one-way transfer makes more sense.
There is a third arrangement: intercity transfer. Going from Istanbul to Sapanca or Bursa and back the same day is not a one-way transfer but a full day's work.