It's funny that 3 years ago no one would doubt Halo beating Unchanted in sales. But ND has gotten very popular over the last few years. Not to mention the PS4 is close to 2/1 XB1 in sales.
It's hard to tell, there's been mistrust in i343 regarding Halo4 and MCC and the fact that a big portion of Halo 5 you dont play as the chief will dishearten fans.
Since this is 3 months ill lean towards Halo.