It depends and it has been debated a great deal in this forum, so reading previous posts is a very good idea.
Passing the JDK1.4 version of the exam is probably a little easier than passing the JDK1.5 (Java 5) exam for two reasons,
1) The 1.4 exam is just a little easier 2) There is huge amounts of study material available for the 1.4 exam less for the 1.5
However, the 1.5 exam covers important new features in the language and if you want to prove you are up to date with the latest trends then you might want to go for the later version of the exam. But as I said, checkout the previous posts.