If you shoot video then the articulating screen of the 600D (and the new 650D) would always come in handy. The savings could be used for a nice spanking L lens later on, they are a much better investment than the camera body.
The cost justification is entirely up to the user. I have used both these cameras (I always get sent Canon cameras to test), and the build and the handling of the 60D is what seperates it from the 600D. It's a beefier and sturdier camera which can be used for hours, and in rougher environments.
