Yes, they are very helpful if you can understand and apply the principles. The Art of War and the Book of Five Rings are not books that you can pick up and automatically understand thoroughly. You pick up on little bits as time goes on. You meditate on the concepts and you see ways that it applies to you and what you've done in the past. I realize something new every time I read them.
If you're a deep-thinker that likes to meditate upon concepts, then I say get them both.
And I'm not trying to advertise here, but you don't have to buy the books new at a bookstore. There are several online stores that sell used and new books for quite cheap. abebooks.com has a search engine that will direct you to book dealers. It's basically like the google of book searches. To the best of my knowledge they have more books listed that ANY site.
"One Mind, Any Weapon"