basically the same except the frequency.

When they say NextG they are talking about the Telstra only 850 MHz 3g network which operated simultaneously on 850/2100 mhz and also uses HSDPA. 850 MHz is the same frequency as the CDMA network which provides greater distance coverage due to the lower frequency used.

The "3G" network is the 2100MHz "city only" 3G network which doesn't give the same coverage.

had a little help from google there