Samsung will stop updates and support for the S3 while Moto G will get the next couple of Android updates fore sure. Even if you use custom roms its an advantage that Moto G will get stock updates
Just to let you guys know, I just woke up and pushed my Moto G to the floor (hard floor) today. It's the third time I let this phone fall and it does not broke. Not a single scratch
i would say s3 has better camera but that is about it.
moto g is snappy and has great batterylife and screen. also moto g will get 4.4 in january so my guess is that it will also get the next version of android.
I had an s3 for about one year and sold it as soon I got the Moto g.
The s3 is a nice phone but the battery use to drain fast sometimes and have a lot of software issues. Also, even with cyanogenmod it freezes sometimes. The screen is bigger but not that much and in comparison with the moto g it looses in brightness.
GPS and Google now consumes a LOT of battery on the s3 while in moto g they barelly appear in the battery graphics consumption. Also, the moto g have a better design and feels great in hand, while the s3 is kinda ungly and you will need to use it with both hand in lots of situations.
I'm in love with my moto g and selling my s3 to buy it was one of the best choices I made. It runs android flawless!!!