For a long time in education we have used the term "comparing apples to oranges" usually in the terms of yearly state testing.
Now Google allows for nutritional comparisons for food showing the searcher calories, fat, sugars, and the rest of the nutritional values.
Go to your Google search box or the omnibox.
Type in Compare X vs Y, with X being your first food and Y being the second. I found it likes plural terms better (which is weird because usually Google searches like singular search terms).
It didn't produce any results for Big Macs vs Whoppers, but maybe they are working on that. It did show results for hamburgers vs hot dogs.
Could be a good tool in the classroom for decision making, nutrition (since schools are making the kids fat, NOT), estimation...
One more great resource from Google. Give it a try.