Composite stirrups: pros/ cons/ why choose them? I have ridden in them a few times but didn't know the reasoning behind them?

Also, bought an amerigo dressage saddle (non calf skin) and noticed that the stirrup leathers I have are rubbing on the saddle. Do Amerigo's need a different type of stirrup leather, are the amerigo stirrup leathers the only brand to use on an amerigo saddle?

Thanks in advance.