That's a loaded question, you really can't go wrong with either of them. They both have advantages over the other, display quality, physical size, expandable memory, build customization, camera, available accessories, wireless charging, processor speed, and tons of other subjective qualities.
I'll give you the quick and dirty that did it for me (I came from an iPhone 6Plus):
1. Nexus 6 was too wide for me, bought one and returned it the very next day
2. Expandable memory, SD card slot, and
3. Customizable build
But #1 was truly the deciding factor. Good luck with your choice... do lots, and lots, and lots, of reading.