The 5X isn't large. Wtf. Especially if you've ACTUALLY been using large phones.
It's weight isn't much off the OG 5, has way better battery life, WAY better camera, fingerprint sensor, punchier display, and response is speedier. Bezels could be smaller, but it's a worthy tradeoff vs old N5 for the improvements. It's still significantly smaller than a N6 or N6P.
I say if you were considering the Moto X Pure, the OPT, or Z5C then you absolutely should consider this phone as well. I've done a 180° on it myself anyway.
Now between it and the 6P, it just comes down to size preference honestly. If you can manage a bigger device, then go 6P honestly. If size is critical, seriously consider its smaller brother.
Edit: The 6P panel isn't anything to write home about. It isn't much brighter than a N6 screen outside (though the screen coating means less fingerprints and less reflectivity as a result - so it stays cleaner, better). The white balance and colors can shift pretty noticeably at angles (very similar to the N6). It is better calibrated though. Saturation is toned down a bit and the panel is slightly cooler (which, considering the temp of the OG, should put the N6P even closer to 6500k).