So far, I have yet to see an alternative fuel that is truly "greener" than gasoline. Just because some politician or "save the earth" radical states it is, does not necessarily make it so!
I have sufficient experience with propane NEVER to use it again (I will get out my Schwinn first!); and would not consider ethanol IF I had a choice (which currently I do not, thanks to our government). The E10/E15 is certainly not greener.
I have no experience with CNG, so no comment on these fuels.
I truly believe electric may be the long term solution; not that it is currently more cost effective, but because electricity may be made by many renewable methods.
My opinion, others will differ.
Jon.