Once you get either, Bruce and Jerry's tutorials will help you understand how versatile the MFS is.
I bought the 700 and then the 400 rails separately. That way you have both. As for what other uses....you can use them for making mortices, dados, circle and arch cuts, of all sizes.
I found that once I learned how to use the system, it didn't get put away for length of an entire workbench project. Every time I started another step in the build, there was another place to use it.
Brian