When speaking of local favoritism:
The local pilots tend to know a little bit more about the local geography and are more willing to do tighter approaches. If two similar A/C (one foreign, one local) is going to be overhead the inbound fix at the same time, and I see an opportunity to tighten in the local, I'll do it.
But that's on approach.
On departures I try to be fair, but safe and expeditious flow of air traffic is my main priority ;-)
We have some slow movers at my field, and they have to be let go sometime.
Normally it's GND that makes the departure sequence. TWR may change it if necessary.