I'm replacing a (2014) Moto X with a nexus 6 which I just got yesterday and so far I'm VERY glad i did so. The Moto X is a great phone, but the screen on the nexus 6 is amazing, external speakers are better, and despite what some reviewers have said it seems to be running very very smoothly for me, even moreso than the X. Having an extra row for more icons and/or larger widgets is also nice. (That gargantuan size does have some benefits!)
It also has some of the key things I liked about the moto x (lock screen dimly lights up whenever phone is moved, always listening for 'ok google' even when sleeping...). The other main advantage is the camera, which I'm actually happy with so far, as opposed to just 'ok' with.
It really comes down to whether or not you think the extra resolution and screen real estate is something you want (along with bigger phone dimensions/weight.) and is worth the premium price. For me the answer is a definite yes. Either one is a great phone, and that is a great deal for the moto x. I hope you have a great time with whatever you choose!