Interestingly, Halo 5 is going to hit XBO at the same point in its lifetime as Halo 3 hit X360. That will be the more interesting inflection point as to how XBO will perform relative to X360 for lifetime sales as Halo 3 marked a significant jump in X360 sales.
hm.... On the other hand, I'm not sure the anticipation for Halo 5 (coming off of MCC and Halo 4, previously) is comparable to Halo 2's cliffhanger and seemingly well-loved MP.
The marketing campaign will be interesting to compare at least.