I think you are going about the choice the wrong way.
No point in just saying the Logan is better compared to your present car - it is how it compares with alternatives.
The Logan is a great big lump of car. It looks to be rather short on engine power for a car that size and I do rather question the ability to deliver the mpg claimed. On paper it matches smaller cars but I really think it will not deliver the economy so well.
If you want to consider green credentials then I think you have to look for something that is not the size of a bus. There are some very roomy medium estates around that will deliver the economy. My guess is that they have a VW style tweak on them.
Generally from the posts above you can see that no one really cares about the environment and is happy to buy something that will burn up much more fuel over its lifetime if it saves them a few bob on service costs.
If you are looking at the green options then it is simply down to Diesel kicking out less pollution but what it does kick out is thought to do more harm.
That said, you are buying something with the latest cleanest diesel engine. The tests and pollution concerns are relating to the cars and commercials presently on the road, many of which are now 10+ years old. Has the concerns with diesel already been addressed to some great extent?
Personally I would go with a diesel. My wife has a Pug 208 diesel with a 1.4 in it and it returns great MPG Same engine in the 308 estate gives a decent size car you can probably chuck two bikes in.
Hope you get it sorted.