We have been travelling Australia for over a year and have both Telstra and Optus platforms available.
To help those about to embark on travels around Australia we conducted a random, but typical, 30 day experiment based on our level of coverage from our campsite at the end of each day. We all know there are many black spots on Australian roads where neither Optus or Telstra have any service, so this experiment does not include service (or lack of it) while driving.

We have always been with Telstra for the wide coverage available while traveling Australia. Before leaving to travel Australia full time mid last year we had heard positive things about Optus reaching remote areas where Telstra has no coverage, so we changed one of our devices to Optus to keep our options open. We have had a combination of Telstra and Optus for 14 months.

There have been a handful of locations where we have had Optus only. This is the reason we changed one of our SIM cards to Optus and it has been useful. Mainly through central Australia but also Imintji Roadhouse on Gibb River Road, Bungle Bungles and Karijini in WA, and most recently Cape Crawford and Lawn Hill In Queensland.
Now, Optus deserves full credit for providing these remote communities with communication that we all take for granted. Thumbs up for that! If you live in those remote areas then Optus would be your hero!
We have been disappointed with Optus regularly having SOS only, or No Service in many towns which are not necessarily remote, and we would expect some level of coverage. In the exact same location Telstra often has several bars of at least 3G, and sometimes even very strong 4G.

The results of our random 30 days were:

18 days Optus coverage versus 30 days Telstra 

The frustration of having No Service, or SOS only with Optus, when Telstra is readily available, outweighs the positive of having Optus only in a few remote locations within Australia.
If you are traveling Australia depending solely on Optus you would be negatively impacted. As our random snapshot reveals, even a simple text message can not be sent around 30% of the time! 
We are fortunate to have both platforms available for the ultimate coverage, but believe that Telstra is the premium choice when traveling Australia.

There are cheap options for both Telstra and Optus cover by other providers. These may be a positive for people who live in a particular town, although if traveling Australia these are not the answer for you. Cheaper providers may claim to use Telstra towers, and that may be true, but they do not use ALL Telstra towers. Believe us, when traveling you are on the lookout for those towers! And every one of them is beneficial to the communication we rely on.

Optus is doing a great job at reaching many remote outback towns where Telstra is not available, and this is a real asset to locals, although they have limited cover in a large percentage of the rest of Australia.
If you are traveling Australia, Telstra gives a larger percentage of reliable cover, although, a combination of Telstra and Optus is ideal.

In our opinion, although more expensive, Telstra is the most consistent platform to use if Traveling around Australia.

We are in no way affiliated with Telstra or Optus. 
These results are our personal experience, although we believe typical of those traveling around Australia.