RatherBeFlying ,the turboprop is only cheaper to run if there is any way to check the compessor blades for damage inside the engine and the Dart and engines of that era there was no way that could be done with the engine on the wing ,it was a workshop job as it had to be stripped down compleatly .modern Boroscope testing with fiber optic cameras through inspection ports did not come in till the 70s and that was pure jet engines [ I can only speak for BEA but we were pretty well up in the fore front of civil aviaton technics] but I'm more than willing to be shot down in flames