If you have the Version D, don't get the G. Differences ain't that much. Just got my hands on it and surprisingly it feels quite solid.
The focus ring is moderately smooth and at f1.8 wide, it's sharper and smoother bokeh than the version D. (own opinion) However, I don't see the point in spending another $300 for the same lens which only has slight differences. Unless you have a non AF motor in your body, it's a different story.
Conclusion: very light weight, feels solid but still it's plastic. AF speed is much faster. Sharper and smoother bokeh.