I understand the basic differences, I think, just based on the titles alone. But more educated opinion would be appreciated. Particularly based on the cost of the resources.
Or, maybe they are just so different that you would recommend both.
Thanks for your thoughts,
Michael
EDIT: Oooops... I forgot the resources. 
A Theology of John’s Gospel and Letters: The Word, the Christ, the Son of God
The Writings of John: A Survey of the Gospel, Epistles, and Apocalypse