They are all very good but it depends if you want a airport or a whole region.
The new airports (the higher priced ones) make use of the new tech that Orbx has been working on like People Flow moving people going about the airports loading the bags ect.
Nature flow moving trees in the wind & leaves falling from the trees.
Vehicle Flow custom cars that drive up to the airport ect.
They are more for the GA low & slow type of aircraft otherwise you would miss all this.
But with Orbx you get lots of add extra things for free like AI Traffic for the whole of the AU regions & free airports.
http://www.fullterrain.com/freeware.html
So back to your question I would buy a region first & then add airports to it I have the whole of Australia & Port Macquarie airport + all there freeware airports.
You can also get a freeware pack of over 100 airports/airfields for Australia.
You can try two large demos of there scenery here
http://www.fullterrain.com/product_demos.html
But like I said they are more for the low & slow flyer.
I am torn between getting a new region as I feel you get more for your money or to get a group of airpots closes to each other so I can hop to & from them.