Thanks to Coop for pointing this out, along with an excellent article in the latest issue of Australian Aviation.

As I suspected, there is more to the selection of the Super Hornet than mere numbers. The F-111 always relied on its performance for protection, flying ultra low level at high speed, the long range affording it the ability to attack a target from multiple directions. It's clear now that RAAF has less confidence on pure performance as a means of defence, requiring the F-111 to be escorted to a target. Furthermore, the F-111 is virtually blind once in the air. It has no air-to-air radar, little in the way of threat warning displays or electronic counter measures, nor is it 'networked.' The Super Hornet has all of these features, even if it cannot match the F-111's performance. Oils ain't oils, it seems.

Although the F-15E has, at first glance, superior performance to the Super Hornet in terms of speed, range and payload, it cannot match the avionics fitout of the Super Hornet. The APG-79 radar is what really sets the two apart, having a variety of sophisticated electronics tricks. The Super Hornet can also be used as a network node, distributing data to other platforms. And this is why the Super Hornet has been chosen, as it represents a technological bridge between the 4th generation F-111 and the 5th generation JSF. This will also ease the latters introduction.

It has also been suggested that the F-22 would have been a better choice, but the US Congress has voted to ban export sales of the F-22, so it simply isn't available. Yet.