The thing for me is that I know that if I pick up a piano accordion, I will, more than likely, be able to play it (the exceptions being freebass but I suspect on piano accordions, they are a bit like hen's teeth).
With a button accordion, who knows what system it is until you pick it up and try to play it (or is there a secret way to know? They all look the same to me!)
However, as far as learning is concerned, I suspect both button and piano are equally difficult for the beginner.