In most Small earth games there is a decision on whether to pick south america or australia, i have been told by good se players that it is South america and i have also been told it is Australia, any advice?

It depends on the initial board

There is no absolute answer to this.
As OvertForeigner have said, it largely depends on how the starts are distributed.

SA is generally better because the armies used to expand can be transferred to key territories (SAf, ME, MEX, etc.) far quicker than AU. However, keep in mind FTBs and the board since that matters more.

SA is generally better, but on boards with North Africa and/or central america AU can be better.

https://www.warzone.com/MultiPlayer?GameID=24154017 in this game AU was better. But I had no intention of letting him have SA. You need to be wary of the other.

In most games South America will be 1 pick and Aus 2 pick but it depends on board.

Short answer: it depends.
Long answer: it bloody depends.

Short answer: Don't play small earth
Long answer: It's terrible