"In my opinion, the maintenence should be cheaper, because in diesel engine is less parts to worry about"
Wrong. More parts..you need to really study the differences.
The BIG issue is fuel "freeze" at below say 5C...very common in cloud.
Ask them how they keep the fuel above say 5C in flight.
Another issue is fuel cleanliness. Ask them how they keep the fuel clean.
Ask them how the engine is controlled at altitude...say 3 000 metres.