With my moniker on Mudcat - What can I say - Diesel is definitely the option.

Old style rules of thumb - if you drive over 40,000 miles/yr then no argument - Diesel.Under 15.000 - petrol in between - then other factors come into play.

New argument : Toyota have the Carina II, now Avensis, with a lean-burn engine. Compression ratios equaling that of Diesel - which return 50+ mpg, around towns as well as open road.

Modern Diesels : Citroen (and by that Renault/Peugeot as they use the same engines) did have a great 1.9 TD. Posts 500 miles to the tank every time (I drive a 1.9TD Xantia with 100,000 miles on it.)
Volkswagon also have a 1.9TDi with I believe simmilar characteristics.
Citroen's HDi series engines form what I hear are pushing that figure higher - They have recently produced a 1.4l HDi Turbo with figures I believe of 70mpg - that's my next car in 2 years time.

In Ireland - Diesel tends to be less expensive also.So mixed with higher mpg than 'most' petrols - runs at far greater cost efficiencies.

Environmental question. Diesel do not produce more NOx (nitrous Oxides) which are the more damageing of pollutants - Petrols do. Pertrols especially at startup and revving modes do. True the catylitic convertor does reduce nearly all these - but only when hot. Your long journey is OK for that.
Diesel is also produced by less distilling than Petrol, so technically less harmful to environment.
Petrol is definitely quieter - where do drive ? Motorways - than no -issue for Diesel, but you will find noise is not a problem in any diesel under 10 years. (the golf above is actually typical of the era)
Particulates from diesel - again no issues with modern engines, are from unburnt fuels - blocked filters, poor injectors etc. and in fact do less environmental harm.

Service issues - small diesel cars ie Peugeot 205 1.9, eat up brake pads and front tyres due to heavy engine up front. Not the same for the same engine in the larger saloon.
Normal engine service parts : No spark plugs but a fuel filter ? very light trade off.
Engine life on Diesel 300,000 miles with reasonble care.
On petrol, normally less than 150,000.

OK I'm biased - but on a long journey, still I believe Diesel tops out.

Look up 1.4litre HDi info on web see what figures they claim ? ( I just did + it's good)

Enjoy the drive (But get a turbo on the diesel - then the 15-40 K difference swings in favour of Diesel

Diesel