Ciao Amalfi
The dualogic transmission is a "semi-automatic" transmission and is only available on the 1.2 petrol engine, not the 1.4 as far as I know. It is effectively an automatic control system that controls a modified manual gearbox, so there is no clutch pedal. The advantages are that you donlt have to change the gears yourself, although you can do if you want, and it is claimed to be slightly more economical. That said, the conventional manual is a little bit faster in terms of acceleration.
Here is a review of the 1.2 dualogic from Autocar magazine .. .just click on the following link ..
Fiat 500 1.2 Dualogic - Road Test First Drive - Autocar.co.uk
The Multijet is a diesel engine and is a 1.3 litre engine. It is a fine engine, but the 1.2 petrol engine is more than adequate. The diesel and 1.4 petrol cost £1400 more than the 1.2, so it's is probably not worth it over the 1.2 petrol unless you do drive fast, or do lots of miles, or prefer more grunt on the motorway.
As far as road tax goes, the 1.4 petrol is the most expensive:
For 2008, the road tax is:
1.4 petrol £120
1.2 petrol £35
1.3 diesel £35