People often quote "You can't compare apples with oranges!"
Every time I hear this I suppress the urge to retort back and ask, "Why not?"
As long as you have identified the context/attribute for the comparison you can very well compare them (or anything else for that matter)
For beginners :
- you can easily compare the Vita A, B, C etc. content in each of them
- you can compare the Sugar, Iron, Sodium, Cal, Mag etc. content in each of them
- you can compare the price and availability in a shop
- you can compare their weight, density, texture
- you can compare which is easier to grow in a particular soil/climate
- you can compare which is more susceptible to a particular disease/rot
- you can compare what make a better diet recommendation based on a person's condition
- you can compare which of these fruit has more juice for the same quantity
- you can compare which stays longer with and without refrigeration
What the heck, you can even compare which taste you like better!