Telstra is the only mobile phone company that offers a choice of network type.  They have both CDMA and GSM networks. Both are reliable but their coverage can differ. Most other Australian mobile networks are GSM. (Only Orange is CDMA) Therefore you have more options for roaming. Even if Telstra doesn't have coverage somewhere, Vodafone or Optus might. GSM is the most common type of network globally,  so if you travel internationally a lot, you will be able to take your phone with you. In terms of price, Telstra's monthly plans are exactly the same whether you choose CDMA or GSM.

Telstra does not offer cap plans which are quite common among other carriers. If you are a heavy mobile user, cap plans would be to your advantage. Telstra's plan have cheaper calling rates the more money you pay per month. They have 1 or 2 year contracts with little flexibility.  If you talk much less or much more one month, it ends up costing quite a bit.  Telstra's plans are only good if you are consistent with your monthyl mobile usage. Even then, you still have to spend at least $60 per month to get competitive calling rates.