Generally, you can breathe easier while sitting.
The best sitting position is when your hips are above your knees, and they are at their highest point on the floor without putting your feet on the ground. When you are sitting in this position, your lung vital capacity increases and makes it easier to breathe.
It’s also easier for the lungs to clear out excess fluid when they’re in an upright position, which is why people will notice that shortness symptoms are relieved once they stop lying down.