OK, A 9kg bottle of gas x 49.9mj of heating energy p/kg will give you a theoretical 449.1mj worth of heating energy to use.

This doesnt take into consideration any temperature variation factors and wouldnt seem relevant with you being situated in QLD as the gas boils and vapourises at different rates depending on ambient temperature.

So we take your 449.1mj and times it by your two bottles to give you 898.2mj.

Your cooktop should have a mj/h rating on it so for now we'll work on an average of 40mj/h for a 4 burner cooktop.

So 898.2mj divided by 40mj/h should theoretically give you 22.455hrs of continuous cooking.

However, being in QLD you will more than likely get butane rather than propane as it has a higher boiling temp. and isnt suited to colder climates so it then becomes,

9x49.2= 442.8mj times your 2 bottles = 885.6 divided by 40 = 22.14hrs which is bugger all difference really.

As for choice of supply it depends on cost, here it costs me near enough to $30 for a 9kg refill at the servo but I get a 45kg bottle exchanged for $95 delivered and that does us all winter with a room heater.

Work out the price of your gas per kg doing it either way and from there you can calculate price per megajoule or per hour using the above figures which will ultimately make the decision for you. Hope it helps.