To some extent is like comparing apples and pears...
You can buy a revenue ticket on any TG flight you want unless completely sold out (it is just a matter of the appl. fare) which will allow you to upgrade for miles if upgrades are available whereas availablity for award tickets can be very limited/non-existing (Admittedly, I have no idea about availabilty for BKK-MEL, but try finding I class availabilty to/from Europe during high season...)