$650?
A new 5X is under $300. A new 6P is under $500.
And I do not like the 6P. I carried one for a while and it's just too damned big. I've got a 5X on Fi (as a backup device/internet hotspot for when both my residential connections are down), and it's a much better phone to carry around. The 6P is huge, and if you put a case on it, you may as well just carry a tablet.
Where are you going that Project Fi has coverage? I thought it was just T-Mobile and Sprint.