Which brake is most expendable?
Keep your front brake. It does the most work, it will stop you much faster than your rear brake ever could hope to. Take a look at a motorcycle, the front brakes are always much larger than the rear. Whenever you brake, on a bicycle, motorcycle, in a car, more weight is transfered to the front wheels, so the front tire has more traction to stop you with.

That said, sit down a minute and think about what you're going to do if you remove your rear brake, and one day your front brake stops working while you're going down a hill. Right about then you'll wish you had just put up with the extra 400 grams of weight.