Petrol Generators is normally high speed generators, this means that it runs at 3000rpm. With this it is smaller, and lightweight, and easy to move around, some bigger models also have wheels.
Another positive look is it is cheaper than most diesel models to buy. It is good for use as a manual generator for short periods of time. Maintenance is not too expensive for renowned brands.
Petrol generators downfall is fuel consumption, especially on the larger models, also fuel storage, and it is also not too good for a petrol generator to be stored for a long period with fuel still in the carburetor, as petrol tends to gel and block the main jet. The carburetor than needs to be cleaned by professional personnel.
Diesel generators is ideal for automatic use, especially because of the diesel fuel, it is fuel injection and this means that starting is not a problem in a longer period. Generators needs to be started often, petrol and diesel models.
Diesel powered generators is expensive, even the high speed generators.
Diesel generators is fuel efficient.
My opinion, if you can afford it, and your requirements is power generation for long periods, than diesel is the answer.