My experience is like this:
- Tactical part is deterministic.
- Globe part is random.
I normally do not replay a mission exactly the same after I load it, but the aliens are exactly at the same locations and sometimes, if my actions are close enough, it seems like they are behaving the same. There still might be some randomness involved in the way the act. There is definitely randomness in the shots and damage, and that eventually affects theirs actions as well.
As for the globe part - it is clearly random every time. You will get the next attack proximately at the same date, but the number of UFOs, their location and actions are different every time I reload a game.