Leopard is the better operating system, no question.
Whether you need it is really only a question you can answer, if you are happy with Tiger and it does everything you need then by all means stick with it.
TBH I didn't need Leopard, but I like having the latest OS and I feel it runs better on my computer than Tiger. App launch speed is better, for example and the Finder is faster and more efficient. I certainly didn't need it though, just wanted it.