Argument is when you want to prove your opinion or point at the cost of winning over the situation/person you are arguing with. However, discussion is more of an openness towards considering the other persons opinion as a possibility in spite of it being contrary to your opinion. Arguments may end up in fights, where as a discussion helps us to be more open,tolerant and indicates respect for others opinion. Discussion gives openness towards a healthy relation and willingness to change your opinion based on reasoning.
Another important point is that discussion helps to develop mutual respect whereas an argument often leads to trying to put down or degrade the other person or their opinion.
Wishing you many more healthy discussions!