And the difference will be how much more once you've considered an extra lot of car insurance (and only one set of NCB between the two) ?
Anyway, as economical as they are, AX 1.5D's are bloody low powered little things (though they do weigh two tenths of f*ck all too) and if you're after a banger that'll be stupidly economical, something like a diesel Maestro is a far better bet. I've got one which I sadly don't have a use for. It'll easily do 55-60mpg, and is in bloody good condition. It's a Maestro van though.
Peter