That is a question that is a lot harder to answer in US than it is abroad, but if you feel you don't need a cutting edge phone in terms of performances, and you're rather intrigued by the price that is half as much as the Nexus 5's prices, then the Moto G sounds like a good choice for Americans, too.
But where I think Moto G will really shine is in countries outside of US, where most people tend not to buy phones that cost more than $300 off-contract. Many even like to buy phones that are free on contract, but with low contract prices, too (meaning the phone itself can't be too expensive).
That's where the Moto G will truly shine, since the price point is so flexible that it could be offered for free on contract, but also off-contract for a price that is much more approachable than Nexus 5, which actually tends to cost around $600 in Europe, unless you can get it directly from the Play Store in a few countries, but even then it's about $470. That makes the price of the Nexus 5 much less appealing in Europe or other places than in US.
There are some differences you need to keep in mind when deciding which is better for you.
The difference in performance should be pretty large between the two. The Nexus 5 and its quad core 2.3 Ghz Snapdragon 800 (along with KitKat) is often described as "stupidly fast", while the Moto G with its quad core 1.2 Ghz Cortex A7, should be several times slower, but it should still have adequate performance for most things you will be doing on the phone.
The Moto G uses an Adreno 305 GPU, which should be roughly 3-4x less powerful than an Adreno 330 inside the Nexus 5. But again, it should be enough to handle the 720p resolution and play all games, which is something Motorola also said at its announcement.
The Nexus 5 comes with a 1080p 5" display, so it's larger and has a bigger resolution than the 4.5" 720p Moto G display. But for most people the size of the Moto G might be preferable, and the resolution is high enough to be "retina" already.
The Moto G comes with 1 GB of RAM (versus 2 GB of RAM for the Nexus 5), which I think is enough for a phone at this price, especially when its KitKat update will arrive in a few weeks.
The Moto G comes with 8 GB by default, and 16 GB is only $20 more, and I'd say that's preferable, since you won't have access to a microSD slot. You also get 50 GB of Google Drive storage (for 2 years). The Nexus 5 gets the same deal, and it's $350 for the 16 GB version and $400 for the 32 GB version (in US).
The battery is slightly smaller on the Moto G (2070mAh vs 2300mAh), but this shouldn't be a problem, as Cortex A7 should be more efficient than the Snapdragon 800, since it was designed to work that way. Motorola says it will have an "all-day battery life", and I believe them.
Unfortunately, the moto G doesn't come with LTE, but considering the markets where it will be selling the most, LTE requirement won't be necessary. But if LTE is something you can't live out with, then the Nexus 5 is the way to go here.
The Moto G comes with only a 5MP camera, which again I find a little disappointing, and personally, I would've preferred they started at $200 if it meant getting an even better 8MP camera. That being said, the camera seems pretty good, and Motorola praised it, too. It's probably a little worse than the Nexus 5 one, though much more consistent, so it should get the job done for most people. It can record in 720p with both the back and the rear camera, and it can also do slow-motion video (presumably 60 fps only).
Ultimately, I still think the choice will be made on price, as long as you believe that the specs the Moto G has are "good enough" for you, and you don't want to pay much more for a Nexus 5, especially if you live in Europe or outside of US. What is unclear is the update situation compared to the Nexus 5, but seeing how Motorola talked about how other OEM's tend to forget about their customers after a year, I'm optimistic we'll see at least an 18 month upgrade schedule for this devices, too.