Need to replace Master Cylinder and Brake Booster. Which should I replace first?

Replace both at once, if you have the master cylinder off, you're halfway to replacing the booster anyways. Pull the check valve off of the vacuum line and try to blow through it both ways. If it only allows air to flow one direction, it's fine. The fact that your engine revs up when you press the pedal tells me that the diaphragm inside the booster is leaking badly, so it needs to be replaced anyways. Good luck!