Or the secret third option, FireFox's built in dev tools! Hit Shirt+F7 to open it. This thing is similar to FireBug, but I like it's stylesheet editor way more (syntax highlighting, etc) plus it's lighter weight.
I still use FireBug for certain inspecting tasks, as it still does some things better (like show padding and margin on the screen).
Chrome Dev Tools doesn't allow you to edit the stylesheet in text mode, which is a major feature I need! Chrome has a much better console, so it's superior for debugging scripts.