Depends on how long you expect to have it and want support. This is likely the last update the 4s is seeing. The Lumia will see longer support from Microsoft.
Depending how well it currently runs I was thinking of picking up a used 4s as a small camera wifi device. If I can find one at good price. Not a ton of options for a compact Internet device with a good camera, and I've been looking.
Over the Lumia? Probably the 4s. Over the G? Most certainly not and this is coming from someone content with his 4s. I doubt that you'll be able to get a 4s under the price of the Motorola G and considering that the Motorola G is more recent it hopefully will be supported longer (hard to say with the Snapdragon 400 while the CPU is significantly better the GPU isn't). Helps the fact that the cheapest 4s will have 16 GB of storage while the Motorola G has a SD card slot and a bigger screen. Though this will highly depend on what you'll be mainly using the device for and what other Apple devices you have.