Leishmans book is specifically aerodynamics based and is a lot more technical. Much more maths capability is assumed on the part of the reader. Simon's book is great, much more readable and is not limited to soley aerodynamics as it includes some dynamics and performance stuff too.
From a pilots point of view buy Simon's book, from and from a design engineers point of view, read simons book and buy Leishman's! Hope this helps!
CRAN