Put simply, Wireless seems to be very picky with reception, much like early mobile phones (well, they still are in some areas). You can be lucky or unlucky, either way, ensure you can cancel the contract without penalty if you decide on wireless.
Keep in mind too, even straight from the horses (Telstras) mouth:
speeds of 550kbps to 3Mbps, bursting up to 6Mbps in the CBD, metro and selected regional areas and soon you'll be able to get typical download speeds of a blistering (lol) 550kbps to 8Mbps in selected metropolitan and regional areas with a 21 USB Mobile Card
Now they are max speeds. In comparison, Optus cable runs at about 9Mbps (and does so the majority of the time) as does standard ADSL. ADSL2+ runs up to about 20Mbps (dependant on how far you are from the exchange) You can see why so many people are disappointed with Wireless.
In a nutshell, unless you HAVE to be able to use the internet away from your home, or can't get ADSL/Cable/ADSL2+, then don't go for Wireless, especially if you aren't in a capital city.