From having bought a tank and the animal at the same time, and having bought and set up a tank before the animal arrives, I would vote for the second of those two options.
I would feel that at least a week would be preferred. It just is a lot easier to set up a tank without the animal in there, and it gives me plenty of time to find problems and correct them.
Also, depending upon the tank size, it allows me to find a good place for it in the room. It isn't like I just had a 3' x 1.5' flat surface open and reading for a big ol' tank.