thats why I don't like the hydrometers. They are certianly accurate. But there are too many variables involved in my reading it accurately. That's why I prefer the refractometer's hands down. Don't have to worry about temperature of the water, don't have to worry about bubbles on the arm, dont have to worry about the box being perfectly level... just dip the pipet in teh water, put a drop on the lens and read.
hand's down, I would go with the refractometer.
But if the extra cash isn't available for you, take the advice given by many above, make sure it's level, make sure there's no bubbles on the arm and rinse it after every test.