I'll be following this thread because I'm working on the same setup.
My Nexus 5 was getting fussy just about the same time my photo editing computer was getting a little long in the tooth. Got excited about the Galaxy 7S Edge but didn't like it and returned it. Then I tried Project Fi, couldn't work with the coverage but really liked the Nexus 5X for $199 so I kept that. But, the Apple wheels were in motion and I ended up buying an iMac and using AT&T's two-for-one to get my wife and I each a new 6S Plus.
Liked the 6S Plus pretty well but, wanting to keep it in good condition, I ordered a waterproof case. Yikes, my new phone is as heavy as a small tablet and almost as big. And I really like some things about the Nexus 5X, like the convenient fingerprint reader, the very good camera, OK Google, maps and it's compactness. (There's a good chance I'll use this as my backpacking phone. The weight of the encased iPhone is significantly more, and I'm happier to carry the less expensive phone.)
So, instead of choosing one or the other, I'll use both. I'm going to activate the Nexus 5X with Ting and then swap SIMS, depending on which phone I feel like carrying.
I'd like to stay with Apple for messaging and voicemail, so I'll have to figure out how to handle that on the 5X. I'd be grateful for any advice.
(Sounds like I'm going in the opposite direction of everyone else, but I've already ported my Google Voice number to AT&T, so I've lost Google Voice as an option. I never liked it, but maybe it's better now. Anyway, for now, I'd like to make this work with the Apple system, even if it means getting my voicemail and messages a bit more slowly.)