Hi
Personally I would not use an illegal taxi but if you are a driver then best to rent a car as all in the is the most cost effective and gives you the most freedom.
The taxi company on site may be legal and yes is cheaper in comparison to others but to me is still a rip off i'm afraid to say. I must admit taxi fares in Spain are to me horrendous and I Kidd not taxi companies at London Heathrow are cheaper!!