Both are good programs. WhatSizeIsIt is free, OmniDiskSweeper is a pay app. I use OmniDiskSweeper, but it came with a suite of products I purchaced.
I have used Delocalizer to free up space, even though it is not rated for OS 10.4.x.
You might think about a program like MainMenu, Onyx, or the like. They are good at cleaning caches, and maintaing the system. The first time you run them, any of them, you generaly find lost space.
Also check other user accounts, after makeing sure you don't need anything from them. Delete the accounts you don't need.