"Can any one tell me the difference in, and advantages/disadvantages of the silver Kooka vs. Koala ?"
I think Kooks are better. May be to do with mintage.
I think the Perth Mint is restricted to 300,000 x 1 oz. Kooks per year.
I think the 2012 1oz. Kooks have sold out at the Perth Mint some time ago. Some dealers may still have some available.