silvers grow bigger, faster.. You will have a 3' + silver in the time it takes your aussie aro to hit 2'.
silvers are very tame however, and you can keep a wide variety of tankmates.
Aussie aro's, like previous poster said, probably a jardini, are very.. Very agressive. They are the most aggressive aro species in the world. (RTG comes a distant second)
Now i have heard of peacefull community tanks with aussie aro's, i've just never been successfull at it myself.. In my experience even a small 6" jardini will bully and fight with 16+" fish!!
(I had a siamese tigerfish about 16", their an aggressive species themselfs.. I had a large community tank and threw the jardini in there thinking.. maybe these big fish will keep him in line... nope.. jardini and tiger fight all the time, and jardini beat every other fish into submission including a large oscar that could have eaten him!! - I finally sold that aro as it would not play well with others)
Aussies are nice because they have the body shape and fin structure of asian aro's, and their scales are nice, albiet not impressive like asian aro..
So my conclusions : U like asian aro? dont want community tank? Go Aussie!
You want more then just a aro in your tank? tank space not a problem? go silver!