I had a comparable, but not really the same quandary.
In the Discovery, real world economy showed that the petrol wasn't far behind the diesel, and in reality, running costs were very close.
The Petrol was chain driven, the diesel was belt. Within my ownership the belt would need doing, and cost circa £800.
The Petrol had no known major issues, the diesel had all the EGR, Injector, Turbo etc woes.
For me, it just didn't make sense to buy the diesel. In real world, in a Discovery 3 Auto, the diesel (Abit a 2.7 TDV6 engine) only gets around 22-23 on the combined. Some people claim up to 30 on a run, but I'd imagine this is fluke/downhill/reset when up to temperature etc.
The petrol also cost a useful amount cheaper than the diesel, which in itself buys a lot of fuel.