Good question! This is really quite confusing but the ont- prefix doesn't always mark the opposite meaning. It can also mean that something's beginning to do something (eg. ontvlammen, ontstaan, ontdooien, ontbijten, ...). So strictly speaking you could say that roeren literally means 'to move' and ontroeren means more like 'to set in motion'. But in the figurative sense (which is mostly used today) both roerend and ontroerend practically mean the same thing emotionally moving. If there's a difference I would say 'roerend' will cause an emotion which is a stronger and has a more lasting impact.