It used to be that Nexus devices were always first in line, because they're Google products, and the updated OS didn't have to be modified any further to accommodate manufacturer skins (like TouchWiz) or carrier-specific hardware or bloatware. More recently, Motorola has given Google a run for its money, beating them to the punch in some instances--but that's understandable, since Motorola was up until recently part of Google. Now that the Nexus 6 is getting carrier-branded, though, it's possible carrier-specific system updates for that phone may not all come at the same time, because of the abovementioned issues. So if you bought your Nexus 6 directly from Google, you should be ok--but if you got it straight from AT&T, for example, you might have to wait longer.
Anyway, that's my understanding. Someone correct me if I'm wrong!