If the option arises to buy 360 over Xbone, I'd probably go for 360, when dealing with continuing series like AC4. I'm kinda annoyed at the whole no-backwards compatibility thing, seeing as onging series that started in the previous consoles are now gonna continue into consoles that won't carry the previous games.
At least, when releasing the 360, Microsoft had the common sense to support the original Xbox releases of Halo. I mean, they kinda owed that to the game that saved their console. At least then, it was possible to own the entire series and be able to play them all on a single console. That's not gonna be the case when Halo 5 comes out on the Xbone.
Assassin's Creed started on 360 and PS3 so if they continue beyond AC4 and you want to own the full collection, you now have to be sure to own two generations of consoles in order to access them. And what about series that started on PS2 and continued into PS3? What if they continue into PS4? Are you honestly expecting me to buy 3 consoles, just so I can access the full series?
Stuff that irks me, is all.