Depends on why we are comparing them. By all means juice them, dice them, powder them if you want to look at the their chemical composition. But if you have to carry them, maybe weight is the best faqctor. On the other hand I just checked some prices Granny Smith Apples are going for $2.90 per kg at Coles while the Oranges are down to $1.66 per kg, so it is the season to eat oranges!
Research has actually been conducted into this issue. Someone at NASA has successfully compared apples and oranges in a scientific study by using infra-red spectroscopy to compare the infra-red properties of both fruits. Turns out the profiles of each are very similar and the conclusion of the researchers is that the argument that you can’t compare apples to oranges is actually invalid! Warning thought , it is not very robust research, but good fun nonetheless! http://improbable.com/airchives/paperair/volume1/v1i3/air-1-3-apples.html