If you want timely updates over the life of the phone, then a pure Android phone like the Pixel is the only way to go. The Nexus 5X is already on 7.1.2, while my Verizon S6 is on 6.0.1! What a joke. My next phone will definitely be a Pixel. Who cares if the hardware is the absolute best. All of them work fine these days.
A big part of that is Verizon... I have a international S6 and just got upgraded to 7.0. I am noticing a hit to performance... not sure if an issue or just OS perf on older hardware.
Personally I find it pretty horrible that carriers like verizon is no longer subsidizing phones (well not in the traditional ways) and still is controlling software updates etc. IMO the phones they sell can have some verizon apps preinstalled, but they should be uninstallable and should be a standard vendor build updated by the phone vendor.