Where it differs is spf13-vim is focused on cross platform dependency, keeping with vanilla vim feel and having no external dependencies (with the exception of git).
My understanding on Janus is that it focuses on Ruby (spf13-vim supports Ruby well, but Python, JavaScript, PHP, Shell, Scala and a bunch more as well). Janus includes many plugins that depend on Vim Ruby support and is definitely focused on the MacVim platform.
My goal was a Vim configuration you could run anywhere whether Linux, Mac or Windows to efficiently develop in any language.
Hopefully you will find the quality control is good, general feedback is very positive. If you find issues please submit a issue on github or even better a pull request. You can see that I'm very responsive to pull requests.