Diesels and gasoline engines would really have basically the same spark (or ignition) hazards coming from the exhaust pipe in a hazardous environment (hot gas, hot particles would all be similar from an ignition point perspective). Maybe the only slight difference would be exhaust temperatures(?), but once they exit the tailpipe they'd surely be pretty close to each other.
The more convincing argument for diesels over gasoline engines is that it's easier to have a fully mechanical diesel (compression ignition) engine running in a potentially hazardous environment in an explosion-protected state than it is for a petrol engine, which requires some electronics to run (spark plugs, etc) which would be a horror to try and get Ex protection for. There might also be arguments for cheaper fuelling if they can supply themselves either petrol or diesel on a refinery, depending on the plant.