Interesting that "Lisp" is assumed to be Common Lisp and that Scheme is assumed to be strictly a learning tool. I've found Racket to be much more useful than any other Common Lisp/Scheme.
I really only see two reasons to choose Common Lisp over Scheme. The first is that CL has a ton of libraries compared to most Schemes (though Racket has quite a few of its own.. I'm not sure how those two would compare on the library front). The second reason is that CL has a much larger community.
Racket is Scheme.
Scheme is Lisp.
SBCL is Common Lisp.
C99 is C.
C++0x is C++.
C++ is C.
Clang is C99.
Do all the lines seem correct?
LISP 1.5 is like C
Common Lisp is like C++
Scheme is like Objective-C
(ignoring all the other Lisp dialects that have fallen by the wayside: Maclisp, Interlisp, ZetaLisp)
I also agree, that Racket is not Scheme, it's a new language. But I stand, that Common Lisp is still a Lisp. Lisp 1.5 would be BCPL or B :) (Why? Because C is in active use, while Lisp 1.5 isn't, so your analogy isn't quite correct)