Uncharted already pretty much owns Halo everywhere except North America. Halo 4 may be the lowest selling Halo game in North America since Halo:CE (excluding Halo Wars and ODST). Though with digital sales it's probably gone past Halo 2. So it very much depends on whether Halo 5 can freshen up the series. If not then the install base differential will make it close. I expect Halo 5 to sell at least 5 million in North America, and UC4 won't reach that level. So I think it will be close.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix