Default Upgrading from S3 - Moto X or Nexus 5? Upgrade future?

Hi all,

I've had a Galaxy S3 which I've been happy with for the past year and a half, however want to move onto something newer. I'm an off-contract user, so buy my phones outright and then utilize prepaid on various carriers for my service.

Anyway my primary concerns are getting fast android updates, stock experience, and a not-too-large phone (no bigger than S3 ideally).

So I picked up a Moto X Developer Edition - seems like a great phone, very mod friendly etc., very fast, nice size, however I'm concerned about future software upgrades.

I know Moto has been very good about updating the OS to the newest version quickly, however I wonder about the S4 Pro dual-core processor inside - specifically if Qualcomm will continue to make drivers for it as new android versions come out. It's a similar processor to that in the Nexus 4, which is now 18 months old, and therefore technically falls outside of Google's prescribed "update window."

I wonder if the Moto X will get stuck behind on software updates not because of Motorola, but because of its older chipset.

Is this a valid concern? Would the N5 be more future proof in that respect? It seems from a usability standpoint, both phones are top notch and more or less exactly what I'm looking for. I want to own the one I end up choosing for another 1.5 years or so, and want it to remain pretty fast / updated to latest OS over that time.

Thoughts between the two?

Thanks much,
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Phone: Samsung Galaxy SIII SGH-i747
ROM: Stock NE4 Debloated
Kernel: Stock or KT747