The main difference between a sim card from TelAway and from www.usaprepaidsimcard.com.au is that we use the AT&T network, whereas TelAway use T-Mobile.
So what's the difference between AT&T and T-Mobile?
Well, AT&T operates on the 3G 850MHz frequency, whereas T-Mobile operates predominantly on a different 3G frequency network that isn't compatible with most Australian phones (and in particular not with iPhones). This means that you may only get 2G speed on your phone with T-Mobile unless you happen to be in one of T-Mobile's limited 3G 850MHz frequency areas.
Coincidentally, Telstra also operates on the same 3G 850MHz frequency that AT&T operates on, and because most mobile phones sold in Australia are compatible with Telstra they are also compatible with AT&T! For example ALL iPhones in Australia are fully compatible with the AT&T 3G network.
To find out more whether you particular phone is compatible with AT&T, please click here.