A Hypothetical Question. What's Harder To Assemble - Predecimal Aussie Gold Or Silver Type Set?

I would rate the gold set as "harder to assemble", simply because there are more rare items to assemble in it.

Here's my reckoning of what a silver "type set" comprises, as per your definition which includes the Holey Dollar and Dump:
- Holey dollar
- Dump
- Edward VII threepence, sixpence, shilling, florin
- George V threepence, sixpence, shilling, florin
- George VI sterling threepence, sixpence, shilling, florin, crown
- George VI debased 1946-7 threepence, sixpence, shilling, florin
- George VI debased no IND IMP threepence, sixpence, shilling, florin
- Elizabeth II 1953 faithless threepence, sixpence, shilling, florin
- Elizabeth II threepence, sixpence, shilling, florin
- Commemorative florins: 1927, 1934/5, 1951, 1954

35 coin types total. All easy to come by coins, except for the first two.

Now for gold, including the Adelaide pounds we have:
- Adelaide Pound Type I
- Adelaide Pound type II
- Sydney Mint Type I ½ and 1 sovereign
- Sydney Mint Type II ½ and 1 sovereign
- Victoria young head St George 1 sovereign
- Victoria young head Shield ½ and 1 sovereign
- Victoria Jubilee head ½, 1, 2 and 5 sovereigns
- Victoria Old Head ½ and 1 sovereign
- Edward VII ½, 1, 2 and 5 sovereigns
- George V ½ and 1 sovereign

21 coins total. Fewer coins, but not only is each individual "common" coin more expensive due to the metal price alone, but there are more rarities: both Adelaide Pound types, as well as the four multiple-sovereign types, giving us six very-high-priced hard-to-get flagship coins to chase down, compared to just the two silvers.

Even if you exclude the "colonial" coins (Adelaide pounds and Holey Dollar/Dump) on account of their being issued in the name of specific colonies rather than "Australia" as a whole, the gold coins still become the "hardest" set to complete simply because of those rare multiple-pound coins from the Sydney mint in 1887 and 1902. Of course, one can in theory exclude all of the gold coins except for the Adelaide Pound and Sydney Mint types on account of their technically being "British Empire coins that happen to be struck in Australia" rather than "Australian coins". Even then, I think the two Adelaide Pounds are harder to get than a Holey Dollar and Dump. Holey Dollars have a higher estimated survival rate (some 350 or so) than Adelaide Pound types (about 40 and 250).

Note that we're also excluding the Adelaide 5 pound coin from the list, on account of it being semi-mythical and only known from 20th century restrikes from the original dies.

I did forget to add to the silver type set the four commemorative florins; I've just edited them into the above list. But even the most expensive of these (the 1934/5) pales in comparison to the holey dollar; all four of them are relatively easy to acquire both in time and in cost.