A few things to consider... both airlines hired thousands of pilots in the last several years.

Do you think you'll see those SYD layovers? As a captain? Or would it be better to have flexibility to make your own schedule and go play overseas on your own time and your own dime because it's easier to throttle your schedule?

On the other hand, Southwest is a grind. It's a domestic airline pilot grunt work with much nicer Hawaii trips than the legacies because we don't do redeyes from there, unless redeyes are your thing, in which case it's United hands down.

It really boils down to what you want, what your priorities are, then roll the dice and you'll find out if you made the right decision the day you retire.