plaster first.
One reason is that plasterers work down as low as they can with the skim, and the screed then comes up above the line at the bottom, and you get a good clean line.
This isn't possible when the floor is in first - it's very difficult to plaster down to a definate edge cleanly - stopping the trowel at the floor makes it hard to finish. Also, when they 'wipe' upwards with the trowel, and the floor is in, it's inclined to bring dust/debris from the floor up into the plaster finish.
Plaster first, my 0.02