The point where it changes gear. Normally a Sport auto will hold gears longer, use more fuel and won't be as relaxed to drive, but these boxes give you the choice of 'Sport' or 'Comfort' settings. I'd be worried about putting £50,000 plus into a diesel in the current climate. I'd also be worried about not getting it before April when the draconian new tax regime starts because, after the initial CO2 based first year tax, you're going to be in for £450 a year whether you go for petrol hybrid or for diesel.