Spot the difference

One of these stamps is much rarer than the other.

They are both listed in the Stanley Gibbons British Commonwealth catalogue -  however, one is valued at SEVENTEEN TIMES the other - BUT which one? 

Have a guess and email you answer, giving a reason if you wish, to  eandephilatelic@gmail.com There isn't a prize, other than the satisfaction of being right!

The Solution

The valuable stamp is on the left: