What is the order of tasks when refitting a bathroom?
The advantage of tiling first are that you don't have to cut as many tiles - just those at the room corners and around pipes. Certain areas will be easier to tile too.

The disadvantage is that you need more tiles. You will also need to make sure that all pipes are in the correct location but not connected. This will mean either having the water turned off or fitting isolation valves (of itself not a bad idea).