[Jason D (Jason)]

Richard,
The main advantage of the 2" over the 1.25" is providing a larger view to detect where all the reflections are. When your scope is somewhat miscollimated, reflections will unstack and many of these reflections will get out-of-view with the 1.25" autocollimator which makes it harder to stack them.
A 2" autocollimator does NOT make reflections brighter.
I do recommend using a 2" autocollimator over 1.25" given the focuser supports it.
As far as which 2" autocollimator to purchase, there are few things to keep in mind:
1- Reflectivity of the mirror. You need to be able to see the 4th reflection of the center spot. If you can’t then the autocollimator will not provide the desired accuracy.
2- Flatness of the mirror. If reflections change shape compared to the original center spot then the autocollimator is somewhat useless.
3- Mechanical accuracy if the autocollimator. To test it, rotate the autocollimator in the focuser without tightening any of the focuser’s set screws. Evaluate how well reflections maintain their positions. Little jittering is OK but if reflections shift their positions noticeably then the autocollimator is somewhat useless.

Based on my experience, only the Catseye autocollimator passes on all above items. My Astrosystem autocollimator passed item # 2 but it did not do as well on items # 1 & 3.
In the USA, the difference in price between a 2” Catseye autocollimator and a 2” Astrosystem is only $11 (referring to only the single pupil autocollimator) which is a little extra to pay to get a much better tool. But, I do not know what the difference would be in Australia.

One more thing. The autocollimator theory explanation in the Astrosystems website and documentation is incorrect and their procedure is incomplete and somewhat incorrect. The proper procedure to use an autocollimator is explained in the Catseye website.
Jason