RE: Do I need the 14
Get a mirror and see what the angle of your leg is when you put your foot on top of the 12 vs 14-inch step. If the angle is "tighter" than 90=degrees, the step is too high. I'm 5'6", and I alternate between a 12 inch and a 14 inch step. When I lift heavier, I go with the lower step. I make sure too dip down a bit at the bottom of the move, so that I can engage my glutes a bit more on the push up (through the heel).
Go with what feels good to you. If you feel any discomfort in the knee joint, don't use that height.