Hypothetical antenna ?

If the adjustable whip on a 4' antenna needs to be raised to "x" position for best swr would the adjustable whip for a 5' identical antenna also need to be raised to "x" position to achieve the best swr and would that swr be the same in both cases?
Also would dual co-phased 4' antennas on the mirrors using the RG59 coax kits be better/worse than the same antenna only in the 5' version mounted in the center of the vehicle using RG8.