I was mid-reply when my laptop shut down on me. Hopefully I can remember the highlights of what I said.
I have a Galaxy Nexus. Thanks to antiquated Canadian 3 year contracts (or they were when I signed), I have a ways to go before I have an upgrade. BUT if I had a chance to upgrade, I'd be torn between the GS4 and the MotoX. Here are my thoughts on why...
GS4 still ranks as a top 10 smartphone of 2013 (average place of 6 or so in the lists I read). Considering how many phones were released this year, that's still a very respectable rank. Camera STILL is pretty awesome compared to phones released after it. Plus, considering it was released just about every major market on the planet, it has great support for accessories and such. That may be something to consider. Replaceable battery for me is a good thing to have, and considering that of the 16GB, only about half that is available (thanks to bloatware and TouchWiz), the SD card could be handy as well (BUT S4s seem to have a knack for destroying sd cards... something else to keep in mind). Plus, I think all the extra features of TouchWiz would be fun to play with for a while until I got bored/annoyed enough and disabled them. S4 has also been out long enough that almost all issues have been dug up, patched, fixed, worked around (or at least are well documented). This is a good thing. I wasn't aware of all the problems that the Gnex had -- and I've experienced most of them (and a few unique ones)
MotoX. New, slick, fast, smart. Active Notifications. Almost pure Android (very few additions, and those additions are actually useful day-to-day). Decent camera, especially after the update. Incredible battery life. Amazing reviews. About as slick and simple and "it just works" that you can get in the Android universe (from what I've read). Rave reviews across the board. A few issues popping up, esp related to Motorola's customer support/care. Incredible radio/reception. But since its still fairly new, long-term issues haven't come up yet. Weaknesses as far as long term usability/durability are just an unknown. That's part of the risk of buying a devices that's fairly new on the market.
If it was my upgrade, I'd have to spend a couple days really digging into reviews on both, plus do as much hands-on playing in my local store as the employees would allow. Right now at this moment I'd lean more towards the S4, but its to close to say (for me
If you buy a device that's on the market 6 months or less, you run the risk of buying a product with known quality issues, but it is newer, and therefore slightly more future-proof. Buying an older - as in 6 months on market or more - device you loose some of that future-proof-ness (is that a word??) but also have a better idea of consistent bugs or quality issues
Finally, there's always something bigger, better, smarter, faster, etc just around the corner. That's the harsh reality of mobile tech at this point. Its evolving so fast that an awesome device like the S4 (released in April I think -- 8 months ago) is considered "old". Buy the device you want/need/can afford now, and don't wait for "the next big thing" because as soon as it lands, the "next big thing" will already have tons of rumors floating around it.
Summary: I'd lean slightly toward the S4. Your mileage may vary