500MB should be plenty for web pages.
Optus 3G network is on 2100mhz band, it tends to drop to GPRS often even in coverage areas, but GPRS still works fine it's just slower. Telstra has the best network, with 3G at 850mhz, but their pricing is outrageous.
I personally would go for Optus unless you're sure you can survive on 200MB.
I'm looking at getting an iPhone, but I'm in Australia which means no unlimited data so I've narrowed down by options to two different plans with different carriers. One will give me 200MB of data each month and it has nation wide 3G access, the other one I can get 500MB each month but 3G access is limited to the capital cities and large towns (outside of these areas it goes back to GPRS not EDGE). How much cell data have you used on your old iPhones and which plan do you think is a better deal?