Pretty much what it says on the tin. There articles here and there talking about how Argentina and the USA we the two up an coming "New World" countries of the 19th Century but later on Argentina kind of crashed and the USA became... well... you know.
So since both country were very attractive to migrants in the 19th Century, I wonder how the balance could be tilted more towards Argentina to make it not just attractive, but more attractive than the USA, or, on the other hand, how the USA could be made less attractive to migrants.