Diesel; engines tend to produce far more torque for a given engine size than petrol. Torque is what counts for towing, not BHP. Gearing makes a big difference too, the reason most land-rover products are plated to tow 3500kg is that with low range engaged, they can make the requirement of pulling from a standing start on a specified incline. They are actually capable of much much more, but 3500kg is a legal maximum. To give you an idea, 4.0l petrol Jeep cherokee, and all the V8 range rovers I have owned come with the same towing capacity as my 2.5 oil burning disco.
I barely see any petrol range rovers, let alone ones actually towing, they all tend to be the oil burners. Not surprising, a quick look at Parkers, an Oilburner, 700Nm torque, compared to 625 for the petrol equivalent, and estimated fuel costs of a grand a year less. Why does anyone even consider a petrol one? Doubly so for towing.