Thanks for your input guys.

As Canadian pointed out, yes, my main concerned is to avoid the visa hassle.

However, contrary to the 'obvious reason' mentioned by Peter42, I haven't got any problems with immigration in these countries (or anywhere else in the world, for that matter). It's just that I am travelling on
a Brazilian passport and, although I am welcome in nearly every country in the world without a visa (including the whole Western Europe, for example), the hassle to get a visa for US or Canada is simply not worth it, and therefore I will choose OneWorld instead of *A.

thanks once again

C.