In reality these 'Green' schemes usually are more expensive when it comes to the price of a unit. They are more of a conscience thing to lull you into thinking you are really doing your bit for the environment.
Then along comes a big anti-social chartered special airliner with four jet engines spewing it's carbon into the atmosphere and affecting the environment and ozone layer, just to deliver an Olympic flame (of all things) all the way from Greece......hardly seems like all the others are taking this issue seriously......does it?
If your main intention is to save yourself some cash, then simply buy the cheapest tariff to suit your lifestyle. The gas and electricity is all delivered through the same pipes or cables irrespective of tariffs !