There is ofcourse no objective, definite answer to the question "which is better". Some people prefer Eclipse, others prefer NetBeans. To find out which you prefer, try them both out and see which one you like best.
I'm a long-time Eclipse user, but I've been looking at NetBeans 6.1 and I'm impressed.
There are actually very few things in life where there really is one "best". Usually, it depends on what you need to get done and how you like to do it. IDE's are very definitely in that category.
I'm a big Eclipse fan, but can work comfortably with other IDEs as well. The major advantage of Eclipse is the extensive collection of plugins available for it, including for languages other than Java, like C/C++, PHP, Python and Unix shell scripts.
The big advantage of NetBeans is that it's from Sun, so they tend to put in goodies that help with emerging Java technologies.
An IDE is no substitute for an Intelligent Developer.