Objectively speaking the 6P is the better phone. It's a bit faster, better screen, better speakers, better camera (slightly, mostly due to the better ISP on the 810)... better battery life... but I will say that if I can't use my 6P for whatever reason, I have no qualms about going to the 5X.
Now, the 5X does have some things going for it. First, it is a very comfortable phone to hold in hand... between the size and its weight. I have big hands, so the 6P doesn't phase me much, but the 5X does have its charm. And in day to day use, the last two builds have really smoothed out the wrinkles... even with the camera. Battery performance is good... maybe a tick below the 6P, but being 'only' 2700mah doesn't cause too much of a problem.
If you are coming from a Nexus 5, the 5X would do very nicely for you... but the 6P would as well... it all depends on if you want the extra goodies that come with the higher price tag. I thought it was worth the premium, but then again, I ended up getting both.