Motorbike? Honestly - how many times is there just you in the vehicle?
I currently have 0-60 in 2.5 secs, 190mph, 45mpg. Cuts through traffic, allowed in lots of bus lanes, road tax is £60, insurance is £180 fully comp (group 20). OK that's a 1200, but a 600cc bike will still do 50-60mpg, do 0-60 in around 4-5 secs and 140-150mph, so cruise all day at motorway speeds.
Obviously I commute a fair distance, but local commuting? It's easy to get over 100mpg from some scooters.
But - if you want to feel really green about it, how about adding in less damage to the roads, less emmisions, less enviromental impact from servicing because of the smaller engine, and less environmental impact on manufacture again because of size.
and it brings good photo opportunities