Hi Karen, the train and most buses ( Greyhound and Firefly? ) follow what could be described as an inland route and of the two I would opt for a train any time if the timetable suited re seeing the countryside, you having more leg room in a train and much more room to get up in and have a stroll to flex the legs.
The inland scenery once past the Blue Mountains is not so inspiring - lots of rolling hills or plains etc.
Another option you may want to consider if you have some extra time is go via the coast, you able to get a regional train that heads south from Sydney to a place called Bomaderry, it being on the northern side of the Shoalhaven River and the town being serviced is Nowra on the southern side.
From Bomaderry/Nowra you can travel by bus down through NSW south coast towns and around into Victoria via Lakes Entrance to Bairnsdale and then from there by train on to Melbourne, the first section of that trip a real rattler of a ride and again for the trip all the way through what we call Gippsland it is not such an interesting journey.
You may need to do a bus exchange at somewhere like Batemans Bay, Narooma orMerimbula all pleasant seaside towns and timetable may mean a stop overnight.
You can find details of a few options on http://www.railmaps.com.au/stationdetails.php?StationSelect=1780
The timetable for Batemans Bay to Bairnsdale/Melbourne is shown on https://railmaps.com.au/routedetails.php?RouteSelect=335&traveldate=2017-08-08&tstyle=