It's not even a prediction that Fallout 4 will sell better. Going by sales, it will be an absolute slaughter.
Going by meta, I'm expecting about mid 80s for Xenoblade and low 90s for Fallout 4.
As for whether I'm picking either up, I don't know yet. I know I'm not getting Xenoblade because of the simple fact that I don't own a Wii U, but I'm also very, very hesitant to buy a Bethesda game day one.