The New Art of Defense in Chess vs The Art Defense in Chess Is The New Art of Defense in Chess or The Art Defense in Chess better? I am 1900 on an upward trend.
Yes, the new book is quite different to the old one:


  On the whole, however, The New Art of Defense in Chess should be seen as a fundamentally new book. This is because Soltis recognizes the way in which dynamism and activity have become fundamental to modern defensive techniques.


Source: ChessBookReviews, emphasis is mine.

Synthesising the review, the reasons why the book is different is because:


Computer analysis has helped players find flaws in the attack in postmortems, which they use to improve their defensive technique
Players are more open to playing 'ugly', counter intuitive moves that are based on concrete analysis
After Tal's reign as world champion, players realised that active defensive is usually better than passive defence