Tom's Hardware did an article on them here:
http://www.tomshardware.com/news/evga-supernova-g3-psu-fan,32995.html
The main difference is that they are smaller, so they'll allow more space inside of your case. I purchased the 750W G3 for my build recently and it is quite compact which I like. However, it was only 10 dollars more than the G2 when I bought it. For a 30 dollar price difference I would have went with the G2.