Is this a survey question as a matter of interest? Or are you actually interested in buying one?
I personally would go with the Nismo out of those two. The Nismo is not actually that different to the stock GTR. They made a few slight changes, including full Nismo body kit (front bonnet lip, front bumper vents, Nismo sideskirts and Nismo Back boot lip), No rear windscreen wiper, no ABS and no AC as per standard. They also included slightly larger turbos (which are laggy and too large to be efficient at 0.8 bar). These were mostly to modify the aerodynamics of the R32 GTR for Homologation for Grp A racing. Grp A cars were not supposed to look the same as the factory car, which included spoiler kits etc. In stock form, the Nismos were not really any faster than standard R32 GTRs.(See youtube below)
However, in saying all of this, everyone knows that the Nismo is the base for such historically classic race cars as the Calsonic GTR etc. Even though the Nismo has more units produced than the ADM, 500 (plus 60 odd that went directly to grp A teams) vs 100 for the ADM, internationally the demand for Nismos will outstrip the ADM 1000:1. So that should mean that the number of buyers competing for the few Nismos remaining ( I assume that many of them have been raced and crashed), should always mean that the Nismos command a higher purchase price.
As im a kiwi and not Australian, i have no real interest in the ADMs at all, so maybe i am bias. Aussies will have to jump in here and give their points of view too.
Possibly a better question is ADM vs Nismo vs N1?
N1s only had 228 units produced, vs the 500 (plus 60) Nismos, vs the 100 for the ADMs. Unlike the Nismos (Group A), the N1s were for Group N endurance racing (off the show room floor racing). In the early 90s, Group N racing in Japan was very similar to Bathurst in the early 70s. You buy a car from a manufacturer, make very limited mods (tyres, brakes, suspension, exhaust and computer i believe) and then race it! So the GTR N1 was a semi prepped race car from Nissan. So the chassis is more rigid, engine built by hand, bigger turbos, lighter everything etc. Of the 228 R32 N1s Built, they can be split into 3, N1, V Spec N1 and V Spec II N1. To be honest i am not 100% sure of the exact break down in terms of quantity, but i believe that most of them were the initial N1 spec and the V Spec N1 and V Spec II N1 number approximately 50 each.
From what i can tell from my general observations, N1s sell for approximately 25-30% more than Nismos in Japan. This is simply my general observations over the last 3 years. It is very hard to work out accurately with such a small number coming up for sale and condition varying so much.
So in my mind, the wish list order in terms of both value and desirability for an Australian wanting to buy an R32 GTR should go like this (all things remaining equal)
1 - V Spec II N1
2 - V Spec N1
3 - N1
4 - Nismo
5 - ADM
6 - V Spec II
7 - V Spec
8 - Stock GTR.
Of course this is just my opinion and everyone is different.