for the extra 3k you get much better seats, better wheels, a better steering wheel, a 3.45LSD, slightly different suspension angles at the rear for a lower roll centre, a "falcon S XR6" badge on the boot thats NOT fake, probably also ABS, stiffer front anti-roll bar, factory suspension that sits at king spring 'low' height, red rocker cover with tickford wings cast into it, a head with a tickford 'T' cast into it (that already flows a good amount of air) and probably some stuff i have forgotten.
the bonus you get from buying an XR6 is if you want to upgrade stuff (diff, head, cam, wheels etc) then its usually pretty easy to find someone who wants to buy your 'old' stuff, therefore making the 'actual' cost of each mod a bit cheaper than otherwise.
i reckon its worth the extra cash, but then, i would, wouldnt i?