I have rooted phone with CyanogenMod android version 5.1.1. If someone already don't know, great app to tweak your hardware is Synapse (you can slightly overclock processor, change battery voltage and many other things).
Hi! Both smartphones are great, but if I had the opportunity to choose I would take Nexus 6P. Nexus 6P have large screen, AMOLED display, better RAM and probably high quality camera.