Hey bro, Android Central had a similar post for your question:-
Should you upgrade to the Pixel XL from the Nexus 6P? | Android Central, while it's not direct answer to your question, but you can do the comparison which one is much more suitable for your requirement, or you might just Google Pixel XL vs Nexus 6P and there would be many relevant post for your question.
But to answer your question, Pixel will be more Google experience than the Nexus series, most of the latest Google functionality will reach Pixel device first (or maybe only available to Pixels). Both of them will be getting the latest Android updates, but the Nexus 6P had only left another 1 year period for receiving the OS updates.
Hardware wise, Pixel are rated IP53 for water and dust resistance(more like a water splash resistance to me), while Nexus 6p doesn't have it. You won't see any big difference in between 3GB RAM and 4GB RAM as both of them are running on Stock Android, which mean you will experience fastest and purest Android experience out of any other Android phones.