laffinboy,
Not a bad question. The type of operation determines whether a turboprop or a turbofan is used.
For short journeys, ie approx 60 minutes or less, a turboprop is much more efficient. It can (for a similar class of aircraft) get to it's optimum cruise level faster and it is usually much more flexible in the approach segments too. The biggest advantage is that they can operate on much shorter runways.
For a long trip, the turbofan comes into it's own. It can cruise at higher levels for a lot longer (and overall faster).
I wouldn't like to cross the Pacific in a turboprop however, I wouldn't like to try to get into an 800m strip in a turbofan.
There are economies of scale with regard engine size too. With a turbofan, the "engine" itself is reasonably small, it is the "fan" that makes the engine appear large.
Again, this is a very simplistic view.