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MIT Scheme is the original implementation. It's built to run code from the REPL and the built-in Emacs-like Edwin editor. It doesn't run scripts from the command line unless you pipe the code into the REPL. It's the only Scheme capable of running scmutils, a powerful computer algebra system written by The Sussman.
Guile was commissioned by Richard Stallman to be the official scripting language of the GNU project. It has a C API so it can be embedded into native programs. It fails in being as flexible as Lua by having way too many dependencies.