The fact that people are actually debating this is kind of silly, but as an American, I also prefer the Leo II. You also have to take into account that the Brits have never exported the Chally II, so there's also another factor.
Everyone is forgetting the French LeClerc! What's wrong with you people!
Honestly the only way we will ever know is when the next generation of tanks come out, and we start exporting M1's Challys and Leos out for unstable African nations to play with, then we will see how well they do vs. one another.
To be honest on the subject on what tank to buy, I was thinking more along the lines of the
http://en.wikipedia.org/wiki/M60-2000_Main_Battle_Tank.
But, alas the Australian Army never had M60's so it's not as good an option. But, it is an option. The US military would love to throw surplus M60's rotting in the mothballs dirt cheap I bet, along with the M60-2000 upgrade (also known as the M60T or Sabra III in the Turkish military) for a pretty low price. You could get way more number wise for the same cost, and the M60-2000 upgrade is on the level of a Merkava Mk. IV, or T-90. So it's still a modern main battle tank by comparison.