Before my current job I was CP of a 135 outfit who did everything in house in Caravans.
I think as a rule the FedEx feeders contract it out and the UPS feeders do it in house. But I'm sure there are exceptions.
In house, in airplane produces much better pilots in my opinion if you're given the freedom to safely explore the capabilities of airplane. I say that as someone who teaches in a simulator now. The flip side is there's some things you cannot do safely in the airplane.
Ideally you'd do a mix of both, but I know of no one who does.