If Plumb is a line pointing directly to the center of the earth and Level is a line at right angles to this, then the line is a curve and plumb is only accurate at one point. However, for all normal purposes, a Plumbline with a heavy weight suspended in oil to prevent excessive swing is the most accurate, and a water level is most accurate over small distances. I allways use a "plumb rule" for upright. (a 6ft length of 4" x 1" yellow pine with a lead weight on a line hanging down the middle in a hole with the line following a gauge mark). The small vertical bubbles in a 6ft level can create quite large errors - half a mm on a 25mm bubble is 30mm at 6ft.