They're pretty different devices... but here goes.
Screen: Unless you really love AMOLED, the 5X is better
Processor: Very close and very similar, I give the edge to the 5X
RAM: 6 has more
Storage: 6 has a 64GB option, if you're looking for only 32 no difference
Size: nexus 6 is huge, i love it. 5x is much more manageable
Feel: the nexus 6 feels stronger, but the 5x feels much nicer
Fingerprint Scanner: 5X wins
Camera: 5X wins
Software: both Nexus, but 5X wins because it'll be supported for an additional year.
Battery life: 6 wins for "heavy users" but 5X did very well for the time I had it. Both easily blow through a "normal day".
Out of 10 Categories: Nexus 5x tied or won on 8 categories. Nexus 6 tied or won on 5 categories.
Of course, not all categories are equal. If it were me, today, I'd go 5X. I only had one for a few days but I really, really, really liked it (replaced with the 6P because I like bigger screens and 64GB).