I've only ever had the flat swag, got this type originally because it could be used anywhere...... rolled out inside the house for a mate to use, used inside a tent and it doesn't take up much more room than a sleeping bag etc.......... however if I was using a swag purely for sleeping outside in the scrub I would be going with the best quality dome type I could buy.
Reason being,
1- Peace of mind to keep the bities out, besides the obvious snakes etc, most parts of Aus do have some nasty kreepy krawlies that can ruin a trip away.
2- Keep the canvas off your body, meaning on hot nights etc u can sleep with nothing touching u, ie; sheet, blanket canvas...
3- Being able to roll over without upsetting the canvas covering you, also being able to roll over without all the noise canvas makes.
4- If it rains plenty through the night and ur in a flat swag, rain is going to find it's way in, unless u have somehow managed to be sleeping with the canvas perfectly positioned for drainage.
5-And for me (being a big unit) u need a lot of canvas to go over me and allow enough to make up the extra to keep rain dew etc away from the mattress.
6 Sleeping in a flat swag I mostly wake up with the canvas thrown off and me pretty much lying flat with nothing between me and the sky except maybe the shirt I'm wearing or sheet blanket, and if it's a damp night or heavy dew this is normally wet.
Buy a dome swag.

Or for the best of both worlds, buy a good flat swag, and one of those swag/cot/stretcher thingos....... that way if u just want to kick the swag out flat, u still can...... if u want to go the full enclosure u can do that too.