I suspect Fallout 4's first week xb1 sales will be a lot closer Tomb Raider's lifetime sales than to its first week sales.
Current amazon sales rankings.
Fallout 4 (XB1) = 19
Rise of the Tomb Raider (XB1) = 103
Current gamestop online sales rankings.
Fallout 4(XB1) = 12
Rise of the Tomb Raider (XB1) = 117
And they release on the same day.