Definitely new tyres on the back.
Yes, the front tyres do more work, but when you find the limit and start understeering, the recovery method is intuitive, you take your foot off the accelerator (or brake) and this shifts weight onto the front tyres, as well as reducing speed, and you can regain control.
If you put the new tyres on the front, you end up with a car that becomes tail happy when you are simply driving too fast or trying to swerve in an emergency. You don’t have the time or space to control and recovery from oversteer, and again the driver would almost always slow down by braking or taking their foot off the accelerator, which further reduces weight on the rear tyres and makes the skid worse!
New tyres on the back, its not a ploy to make you buy or wear out more tyres, it just makes sense.