Contra Chuck, an advantage of Common Lisp is that it has a standard that implementations stick to and strive for, such that you can develop largely with SBCL (which has excellent type checking and inference) and then, say, deploy as an executable linked with C libraries (with ECL or others) or as a .jar making use of Java libraries (with ABCL), or with a Mac or a Windows-native GUI (Clozure, both). Common Lisp is amazingly portable across architectures and over implementations and over time, and Common Lispers make efforts to keep things this way, with the support of the language. As an illustration, one silly divergence of unstandardized behavior is the question "is this a special variable?" So I answered it across the implementations I use:
#-abcl
(defun special-variable-p (symbol)
#+ecl(si:specialp symbol)
#+ccl(proclaimed-special-p symbol)
#+sbcl(equal '(:special t)
(multiple-value-list-int:info :variable :kind symbol)))
which reduces, at read time, to nothing on ABCL (it has this already), to (defun special-variable-p (symbol) (si:specialp symbol)) on ECL, and so on. So I can put this in my .rc files and use the common function at the REPL. But this isn't very important: this isn't threading or variously-backed networking or a Communicating Sequential Processes library. This last example just has one #+sbcl/#-sbcl even as it runs on five implementations. Because it relies on code that's been carefully ported.
But what permits this (and other) advantages also poses its own challenge to the learner: Common Lisp is a very big language. It isn't something you can slurp up in a week or two, like I did Clojure (but my Clojure is already decaying with the breaking changes set to roll out - that language, although heavy with its own merits, reminded me by contrast of a lot of what I like about Common Lisp.) So you should read a lot of this page, and keep the HyperSpec a keypress away (for me, M-x hyperspec RET do-symbols RET is sufficient nearness to the Bible.), and think about buying a few books. I have Practical Common Lisp, just got Let Over Lambda, and will buy PAIP real soon now.
But even if Common Lisp is the True Answer, you won't completely waste your time by 'just picking' some deceptively flashy alternative (-- 'deceptive' because commonplace CL doesn't show you all that its macros can do, and it has more kinds of macros than anybody. The usual comparison is between bad CL and syntax-optimized alternative X). You'll still learn the basics, you can still use much of what you can read in SICP, On Lisp, The Little Schemer, etc. A lisp, even the wrong lisp, is still better than a non-lisp. (But you'll spend some of your time implementing parts of the right lisp, poorly, in the wrong lisp. Meta-Greenspun.)
