Overall, it comes out the winner in being more environmentally friendly.
I have found this to be he case in number of articles I read while researching, and the Huffington Post does a good job of summing up the comparison, you can find it at:
http://www.huffingtonpost.com/2008/06/27/charcoal-vs-gas-grill-whi_n_109524.html
One Change Daily 