Galaxy vs. iPhone
I think the Galaxy S5 might temporarily become the top-selling smartphone, if for no other reason than that Apple and Samsung are on different release cycles, which, as Eric points out, makes sales pattern somewhat cyclical. That said, on any kind of long-term timeline, I don't think think Galaxy sales will be close. What will be much more interesting: If Apple implements the radical design changes that are rumored (e.g. curved class, bigger screens), what effect will it have? I know some people prefer Android to iOS, but the screen-size issue means that comparing the phones isn't quite an apple-to-apples scenario.
The impact of KNOX is anothing intersting unknown. Though iPhones continue to be far and away the most popular handsets in the enterprise, it doesn't seem like businesses have been wowed by the 5S, and if Samsung shows that it's more willing than Apple to explicitly cater to IT concerns, Apple's lead with businesses could diminish.