Really depends on your travel patterns. Normally for someone based in Australia UA Mileage Plus would be better than US Dividend Miles, since you can fly UA from Australia (opportunities for upgrades, elite status bonus, promos, etc).
Whether a US-based FFP suits or not is another matter. Depends on travel patterns. Where do you fly on *A, how much, what class of travel, what do you want out of the FFP, etc.
Since you are young, I am guessing you don't travel much outside Australia and if you do it is on the cheapest fare you can find. In that case there may not be much point in joining any program as difficult to accrue enough to make it worthwhile.