I'd choose the Nexus 5X - mostly because its got a more palatable form factor. I've never been one for huge phones.
Plus, its not much of a trade off as I see it. Same great camera, same fingerprint sensor, still good processor package (storage and RAM coulda been higher, but its not a deal breaker for me). Besides, I loved the original Nexus 5's design and this one just follows suit, so the plastic doesn't matter to me either.